Sigma 18-250mm 79 x 101mm 630g $479
Tamron 18-270mm 80 x 101mm 560g $629
Canon 18-200mm 79 x 102mm 600g $589
I like having a superzoom for walking around and for travel. You can see my photos here:
I will only keep one. I think I like the Canon and Tamron lenses the best so have pretty much eliminated the Sigma. The reviews of the Sigma are a bit worse and it is the heaviest also so that is another strike against it. I think the build quality of all 3 is fine for a lens of this type and price range. For a walkaround lens I do not want a big, heavy, metal lens. Actually, I wish they were all smaller and lighter! There was no lens creep for any lens when pointed down. Of course, when a lens gets older it sometimes loosens up a bit, but all 3 have lens locks. My 7 year old Tamron 28-75mm f2.8 still doesn't have lens creep though.
The Sigma has micro-HSM (fake HSM), but AF seems to not be any faster than the Canon which doesn't have USM. The Tamron AF is a half-step slower than the Canon, but still seems okay. I don't typically use servo AF mode, but a couple of days ago I shot some bodyboarders using servo mode using the Tamron. It was able to keep up and it was interesting to see it constantly keeping the subjects in focus as they got closer. My A700 and 18-250mm could not do that with servo mode. I would always use single-shot mode with the A700 and got most in focus. I also tried focusing on something very close and then something at infinity to see how long it took with each lens. I did it several times. Also, focusing in a more typical situation of subjects at distances that didn't vary so much. All seemed acceptable to me for things I typically photograph. Both the Canon and Tamron AF was pretty quiet. The Sigma didn't seem any quieter. For me, AF noise is a non-issue for all 3 lenses. Focus accuracy was fine with all of them. I have been using the Tamron and Canon the most after I decided I probably wouldn't choose the Sigma. Both the Tamron and Canon focus accurately and without hesitation or hunting in most conditions. Even indoors they both worked well -- except when I tried focusing on my toes in an almost dark room (200mm, ISO 6400, 1/6 second, f5.6) and both could not focus. I didn't really expect them to, but I just wanted to see if one would and the other wouldn't. :-) My A700 + 18-250mm also could not focus.
Tamron advantages: lighter, includes lens hood, photozone.de and dpreview.com rate IQ slightly better, 6 year warranty, 70mm longer focal length, better lens cap (don't need to remove lens hood to put on and off), slightly higher macro magnification
Canon advantages: Canon lens (no worries about future compatibility), AF a bit faster, at 200mm lens length about 23mm shorter than Tamron at 200mm
Note, photozone.de tested the Tamron and Canon on the same 50D body (15mp, 1.6x) so, I think, the results are better for legitimate comparison purposes. The dpreview.com test of the Canon used a 50D, I think, but the Tamron was tested on a Nikon D300 and D90 (12mp, 1.5x bodies).
Note, my Tamron 28-75mm that I bought in 2003 works fine on the 60D, 30D, and 300D. I think compatibility problems are rare with Tamron. I don't know if it is true, but I have heard that Sigma reverse engineers the protocol/interface, but Tamron licenses it.
I also checked to see how the aperture changes as you zoom:
I am not a pixel peeper or measurebator so I have just been taking photos with the 3 lenses on the 60D and looking at them on the computer. No big surprises. They all are okay. I have read several reviews of the lenses and they are good to get the nitty-gritty about optical performance. Of course, we all want the optical performance to be good and when choosing between 3 very similar lenses we want the best of the 3, if possible. As we all know a superzoom is a compromise, but the Tamron and Canon both are acceptible.
By the way, I said that I quickly decided I like the Tamron and Canon lenses a bit better than the Sigma. One reason was because the Sigma was the heaviest, but the main reason is because in reviews on dpreview the Sigma was rated the lowest optically.
For example, dpreview said this about the Sigma sharpness compared to the Tamron:
In a fashion that's almost stereotypical between the two brands, the Tamron certainly appears to be consistently sharper, and has a rather more effective stabilisation system.
Also, dpreview rated the Sigma, Tamron, and Canon like this:
S 7.5, T 7.5, C 7.5 Build quality
S 8.0, T 8.0, C 8.0 Ergonomics & handling
S 9.0, T 9.0, C 8.5 Features
S 7.0, T 7.5, C 7.0 Image quality
S 7.0, T 7.0, C 7.0 Value
The Tamron was rated higher than the Canon and Sigma for Image quality.
Plus, the Sigma does not have HSM, it has micro-HSM (fake HSM). You can read about the difference between USM/HSM and micro-USM/HSM here:
I shoot raw. Lightroom 3 and DxO have lens profiles for the lenses that will automatically correct/improve the optical quality. I don't have either software now, but may get one of them.
I haven't made a decision yet because I want to use them a bit longer, but I am leaning to keeping the Tamron at the moment. Yes, I would like the best points of each of the 3 lenses all in one, but that never happens. So, one is left trying to decide which trade-offs one can live with the best. Now that Tamron has released their new 70-300mm with USD (USM motor) they will probably at some point update the 18-270mm with it, but no one knows when. It will almost certainly be heavier too.
After I bought the 3 lenses I discovered, coincidentally, that Tamron has a $150 rebate on the lens. Cool! With the Tamron rebate the price is $110 less than the Canon and since Canon needs a separate lens hood the price is actually about $150 less. If I decide to go with the Tamron then this is icing on the cake.
By the way, I used to have a 30D and 300D and still have a Tamron 28-75mm f2.8 and Sigma 10-20mm f4-5.6 in Canon mount. For the last 3 years I have been using a Sony A700, but Sony didn't have any new bodies that I am interested in so I decided to get a Canon this time. I will keep my Sony lenses though in case Sony has something interesting in the future. :-) Even if they do have something I like 2-3 years down the road I will keep my Canon mount lenses too. I have no brand loyalty. Nikon and Pentax have interesting stuff too and if I had some of their lenses already then I would consider them. As it is I only have Sony/Minolta mount lenses and Canon mount lenses though and don't really want to get any other mounts. :-)
Speaking just for myself, I have so rarely seen any of my photos that optical quality was the only thing keeping the image from being fantastic. Actually, I love this sort of lens because without it many photos I take would not even be taken. I know because I have used other lenses over the years and I get many more photos that I want to take and keep now than I used to because rather than having the wrong lens on the camera (or wrong film inside the camera) I can shoot at wide angle in one shot and seconds later shoot at a long focal length or anything in between.
Update: I decided to keep the Tamron 18-270mm and sent the Canon 18-200mm and Sigma 18-250mm back.