Well this was a FUN and INTERESTING test. It was PLEASANT using all the lenses as they each handle very well and provide tremendous photographic CONVENIENCE in actual use. Now to some general comments and conclusions...

*IMAGING QUALITY: The Canon 18-55 IS and 55-250 IS combo deliver BETTER OVERALL imaging quality, from CENTER TO EDGE, than any of the superzooms. Maybe that isn't a surprise though. But you do give up the main purpose of having a superzoom (i.e., not having to switch lenses in the field). But between the superzooms, I really like the overall imaging quality of THE TAMRON from what I saw with MY GIVEN samples (I THINK an earlier copy of the Sigma was a better unit than the one in this test). The Canon isn't far behind, and often matches the Tamron, but the Tamron really covers a massive range WITHOUT COMPROMISING output vis-a-vis the others. The Tamron also has the BEST BOKEH of the bunch; IMO much better than both the Canon and Sigma. In terms of FLARE, the Canon does the best job here. All of the superzooms have their ups and downs at certain focal lengths, though the Sigma probably is the worst offender here. Both the Canon and Tamron put out a bit BETTER CONTRAST reproduction than the Sigma, but it's not a night and day difference. The Sigma can deliver some VERY GOOD images as well (and is SHARPER at the WIDE END), but these two newcomers in the Canon and Tamron are a LITTLE BETTER OVERALL.

*AF: The Canon 18-200 IS is really IMPRESSIVE in this category. It's probably TWICE as fast as the Sigma and Tamron in AF speed. That said, I DON'T think you're going to miss shots with the other lenses for the kind of photography these lenses are used for. That is, the AF speed of the Tamron and Sigma are not deal-breakers in my mind, not at all. They are SUFFICIENTLY QUICK to capture kids running around (AI Servo mode), animals playing, equestrian performances, etc. But I did like how fairly SNAPPY the Canon was in this regard. In terms of AF accuracy, I found the Canon and Tamron to be MORE RELIABLE than the Sigma, though again, they all performed good enough. The Sigma did render me a few more OOF images overall though. In low-light, the Canon was the best but the Sigma and Tamron eventually locked on to its subject like the Canon would. In terms of noise, I would rank the Sigma as being the loudest of the lot.

*BUILD/HANDLING: All the lenses are BUILT FAIRLY WELL with polycarbonate shells and solid rubber zoom and focus rings. No they don't feel like Tokina DX Pro lenses or L glass, but they do still exude a good amount of CONFIDENCE in use. They feel much more durable than the Canon 18-55 IS and 55-250 IS, without a doubt. One thing about the Canon build is that it ZOOM CREEPS THE WORST of the three, so the zoom lock switch does come in handy. [Each lens has a zoom lock switch.] But I found it a bit annoying out in the field as I grabbed each lens from the bag as the Canon's zoom barrel would come all the way out as I pulled it from the front element area. The others don't do this. Other than this, I like the Canon very much. Each lens sports a METAL MOUNT so that is a nice touch. In handling, I like the Tamron's VERY WIDE AND GRIPPY zoom ring. On the flip side, I think the Canon's focus ring is TOO SMALL (though not as humorous as the Canon 18-55 IS's). But the Canon's zooming action is very SMOOTH and NICE and DAMPENED WELL. The Sigma and Tamron zooming action is comparable with the Tamron taking a little more of a "break-in" period to smooth out. All lack FTM focusing, so you have to switch it over manually. All in all, not a whole lot to complain about here as each lens is built fairly well and handles nicely.

*STABILIZATION: The stabilization mechanisms in each lens is HIGHLY EFFECTIVE. I can see gains of up to even 6 stops with each lens under the right circumstances (though that isn't the norm). If pressed, I'd say the Canon and Tamron units are just a LITTLE BETTER than the Sigma in stabilizing the VF and minimizing shake. But each one does exhibit a SLIGHT shift or jump upon initial shutter depression (the Sigma the most), then they settle down and smooth things out. The Canon I noticed, with continuous half-presses of the shutter, would sort of make the subject "jump-jump-jump" with each half-press (not unlike the 1855 IS and 55250 IS). But if you simply held down the shutter half-way, this would not happen. The Tamron is overall VERY SMOOTH. The Sigma is probably the "roughest" of the bunch but still very effective in practice. All in all, each one does its job and does it EFFECTIVELY.

*MISCELLANEOUS: (1) Note that the Sigma and Tamron CAN work with the older 10D or D60 models whereas the 18-200 IS CANNOT due to the EF-S mount. In fact, the Sigma and Tamron will mount to ANY body (though on APS-H or FF you will get vignetting at the wider focal lengths). (2) Also to note is the important fact that, at least in the USA, the Canon and Sigma give you ONLY a 1 year warranty while the Tamron gets you a FULL 6 YEARS. That is a SIGNIFICANT difference. Especially with consumer-level lenses using fragile stabilization mechanisms (e.g., Canon 17-55 "IS unit" failure is well documented), having 6 years of coverage is VERY ASSURING. So take this into consideration as well. (3) Moreover, I want to comment on the Tamron's 270mm ADVANTAGE over the 200mm reach of its competition. You can see the difference in my test image, and I think it's a MEANINGFUL DIFFERENCE. If you add a 1.4x TC to the Sigma OS lens, for example, you'll get a 280mm f9 lens. But in the Tamron, you get "built-in" a 270mm f6.3 lens. So that extra and "free 1.4x TC" with the Tamron only costs you 1/3 stop instead of 1 full stop. That's NOT TO BE OVERLOOKED. [And it's an impressive 270mm at that, well at least at the center of the frame.] The Tamron is also at f5.6 at 200mm, just like the Canon, so you aren't losing out at that focal length either. I think this is a BIG PERK you get with the new Tamron. (4) Lastly, both the Sigma and Tamron COME WITH LENS HOODS while the Canon does NOT.

*JUDGMENT [updated 12/31/08 due to pricing changes]: So which lens would I get personally? If money is no factor, I would rank them in this order: Tamron, Canon, Sigma. If money is an issue, I would recommend the Sigma. As of 12/31/08, the Sigma OS lens is *ONLY $350 USD*. That's a steal for this kind of lens. The Canon can be had for about $500, and the Tamron about $575, so that's quite a significant price differential. I used the Sigma OS lens for a while and it gave me some great images. Is the Tamron $225 better than the Sigma? Probably not. But the Tamron costs the most because IMO it offers the most, all things considered. But at day's end, I do think the new Canon and Tamron superzooms are the BETTER CHOICES CURRENTLY overall (apart from price-point).