Sigma makes a much cheaper alternative, their 70-200mm f2.8, and Canon has a smaller lighter version the 200mm prime.  I've included a small AA battery for size comparisons.  Above is a picture of the lenses I'm about to review.

The Lineup
Sigma  70-200mm 2.8     $700    (avg. street price)
Canon 70-200mm 2.8 IS $1700  (avg. street price)
Canon       200mm 2.8     $600    (avg. street price)

I understand it isn't a fair tests, and it is not using professional test equipment.  I'm just simulating what normal people would use this lens for. The intent is to get a general feel for the lenses under somewhat normal usages.

Setup
-No filters are used
-Using Manual Focus on all lenses, so they're kind of subject to what I think is focus (I Will use auto focus in a later test )
-IS (image stabilization) is turned off
-Use tripod at fixed position to prevent shake and to ensure fairly consistent shots
-Lens hoods for the respective tripods will be used
-every shot is shot at 1/250 of a second 
-all shots will be done at 200mm(for equality)
-measure at F2.8,F4.0,F5.6,F7.1 (commonly used apertures)
-all shots taken indoors using main flash, 2 strobes mounted on umbrellas, 1 hot light
-camera used: Canon 30D
-will use 100% crops to get full detail as well as zoom out shots
-shots taken using unmodified original RAW conversions
-focus will be on the words 'global' on the knife' (lower left quadrant, to test versatility, as it is not dead center, most lenses sweet spots)

Questions I wanted answered
1)Are primes really better in image quality than zooms?
2)Is the canon zoom lens vs. sigma really worth the extra cost.

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200mm Prime
Feel: This lens feels very heavy and solid. Noticed it's much smaller and lighter than the other two. The lens hood attaches with a nice click and to me feels securely locked into position. The lens cap is sturdy and fits snugly.

Focusing: This is actually one of the fastest if not 'the fastest' auto focusing lens I've ever used. It's ultra quiet. There is a focus guide to tell if it's 1.3m t infinity or 1.5m to infinity for even faster focusing.

Tests Results: Click on the image for the 100% Crop view
F2.8

F4.0

F5.6

F7.1

Conclusions:  
As expected, I think this lens is really sharp. Not sure how it compares with the rest. Wide open at 2.8, the bokeh is a tad distracting to me. The 100% crop is very sharp. Note even at 2.8, I could see the edge of the blade where the cherries and the blade meet with distinction. For the 100% cropped images, I use the 'Rights Reserved' "R" as a control. at 2.8, the R is slightly fuzzy, but not by much.  As the lens progressively stops down, it gets sharper and sharper. 

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70-200IS Canon
Feel: This lens feels even heavier, and is solid metal. It's white to reflect the suns rays so it doesn't get as hot out in the field. If the lens hood is not aligned right, sometimes it takes a few tries to snap it on.  However, once it does attach with a nice click and feels securely locked into position. The lens cap is sturdy and fits snugly. Zooming doesn't require a lot of effort.

Focusing: Focuses at a decent pace. It  has a switch to tell the lens how far or near the subject will be.  1.4m to infinity, or 2.5m to infinity.  This increases autofocus speed greatly as it has a hint as to where the subject will be. Though again, it isn't as speedy as the prime. Focusing from near to far, it is at a moderate pace.

Tests Results: Click on the image for the 100% Crop view
F2.8

F4.0

F5.6

F7.1

Conclusions:  
Well, evidence here tells me at F2.8, this lens is kinda crappy, compared to the prime.  Looking at the 'R' I remind myself of the 'fuzzy wuzzy was a bear' song. Also note the circles on the handle are a bit fuzzy at 2.8. heck, this thing is so crappy, I'm now actually thinking I didn't even manually focus correctly at all, and almost tempted to redo this, but I'm writing this after the fact, and all the setup has been put away, so there's no way to recreate this again without retesting them all. So I'll pass. Using the strawberries in the back, the bokeh is slightly less distracting, but not by much. At 4.0 the 'R' is better, but not by much. The top edge of the blades where the cherries meet is still a little fuzzy.  It's not until F5.6 that the 'R' is sharp, and the blade actually has an edge. At F7.1 it's nice and spiffy...however, if you do an A B comparison between that and the prime...gee...surprise surprise. I wonder who wins?  The prime kicks the zoom's arse, hands down.  Notice the cherries in the background are sharper and more crisp vs. the 7.1 on the Canon zoom. (NOTE: See amendments at bottom)

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70-200 Sigma 
Feel: This feels about as heavy as the Canon IS version. Though the construction doesn't feel as solid. I think its a metal, but if feels like a softer alloy. Zooming doesn't require a lot of effort. The lens cap feels loose, as if it will fall off any minute.  The lens hood feels as if it has a felt coating, almost like a charcoal texture. It doesn't really snap into place, after you twists, it just closes like a bottle cap and is prone to twisting. Given enough force every cap will twist, but this one requires much less force.

Focusing: Not as slow as I expected.  Focuses about the same speed, if not slightly slower than the 70-200IS. Not noticeably faster or slower, so I'd say on par. (Again, the test is to focus on a near subject, then quickly move to infinity and focus) 

Tests Results: Click on the image for the 100% Crop view
F2.8

F4.0

F5.6

F7.1

Conclusions:  
Well, my first impression is, it's fuzzy. At F2.8, it's about as crappy as the Canon 70-200. Though doing an A and B, I think the Sigma wins on the 2.8 front. (NOTE: See amendments at bottom) Though overall, the colors seem to be more accurate on both the Canon lenses. Note at F2.8, there's less 'silver' on the knife.  What's interesting, is from F2.8 all the way up to 7.1, the 'R's stays fuzzy.  With the Canon zoom, at least at 5.6 stopped down, it's gets sharp and can compete with the Prime. So it wins one battle, but, I don't think it won the war. 

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Overall Conclusions
Well, as I predicted hands down the Prime lens wins.  Rating from sharpness, I'd say Prime, Canon, then Sigma. (with the slight anomaly in the F2.8 region).  I will retest tomorrow with autofocus 2.8 as I'm sure the computer has better accuracy than my lame four eye goggles.  Color accuracy again, goes to the Prime.  Then I'd say the zoom. Sigma has a warm glow to all the shots. And if you notice, using F7.1 as representative for all three lenses, note the color of the cutting board in contrast to the cherries.  The Sigma lens's colors seem to all mush together.  What did I learn from this?  If I wanted to do real personal work, where I can take my time, I will bring that 200 prime and use it hands down.  For event photography, where I do need the flexibility of a zoom, I will take the Canon.  (2.8 aberration aside), the thing is built like a tank has slightly better color accuracy than the Sigma, and generally sharper stopped down.  Whereas the Sigma stays fuzzy throughout.  However, with that being said, is it worth the extra 1k? To someone who uses it for a living, where you need that extra 5% of juice, then yes.  To average people looking to zoom in at birds and to have a good time shooting soccer or sports, I say save the extra 1000 dollars and use a monopod.  That way, the wife can spend more on shoes.  NOTE:   I will retest 2.8 tomorrow between sigma and canon.  I have a hard time believing that it's that whacked.  However, with that being said, Canon does offer a 70-2002.8 (no IS) for 1.1k  I think at that point, were comparing apples to apples against the Sigma.  Build quality goes to Canon, but the slight increase in image quality isn't worth the 400 dollar difference.  I'd say if you're 'that' torn about it, save even more money to get the prime.  If you're shooting soccer or some other sports anyways, I'd imagine you'd want to be at full zoom most of the time.  Plop a 1.4x teleconverter on the prime, and you have yourself a relatively inexpensive, sharp as nails, kick ass lens.

Amendments
Well, I refused to believe that Canon lost to Sigma at the 2.8 region, and I believe it was my screwup.  So The next day I did an 200mm test outdoors with natural sunlight using autofocus. Both handheld, IS off. (Figured for the most part the computer can do it better than my old eyes can at night trying to focus on tiny text on a blade).  The conclusions were more in line with my original hypothesis, though it doesn't change my overall conclusions.  

Conclusions 2.8 Redux
Moving it to a more real world situation, note the fringing on the text '12512'. In addition to slight ghosting on the screws. With this in daylight, I see the bokeh on both lenses are entirely different. Also, the Sigma does appear to be slighter darker in tone, whereas the Canon stays closer to neutral. Click Image to view 100% crop.

SIGMA F2.8@ 1/250 second 

Canon IS 2.8 @ 1/250 second