I've been contemplating on getting the Tamron 18-200 for a few years. The 18-200 was soft at the tele end from about 150mm on, so I opted for Tamron 24-135SP which I replaced with Canon 28-135IS. The 18-250 lens gives you good image quality from 18-220, so it definitely was a no-brainer. It now replaces the Canon 28-135 IS as a one-lens solution. It's that good.
If you're looking for an all-around, travel, have-something-at-that-special-moment lens, this is it. I have a couple of big Canon bodies and huge Canon L lenses in their heavy and huge bags, which I do take to "events" where prints of 8x10 or larger are the norm. But, even the 30D with 24-105L and 70-200L are just too cumbersome to carry around all the time...I used to, but not anymore... So, I bought a Rebel XT (real cheap these days) and put this lens on it....awesome! A tiny $10 samsonite bag holds: Rebel XT, Tamron 18-250, Canon 50mm (in case I need print quality), and a $30 Sunpack bounce flash. This setup (works as a backup too) I have with me all the time! Photography is a hobby and it's better to have something to shoot those "moments" with than to only wish you had the 25 lbs. gear! I've missed a lot of great moments because I didn't have my "HEAVY" gear.
The Tamron 18-250 does exactly what it's supposed to do: One Lens Solution, compact and featherweight. Good IQ (equal or better than kit lens and many non-L lenses), Good contrast and saturation, No focus issues....Let me sidetrack...If you have focus issues, have your body and lenses calibrated. At first I thought the lenses produced front/back focus issues more, but found out that the combination of lens and body is at fault, if not the shooter. Oddly enough, recently my 30D back-focused more than 50% of the time with all 12 Canon lenses I own, but was focusing dead on with 4 different Tamron lenses. I had the body calibrated and was afraid the Tamrons wouldn't work, but now it focuses perfectly with both Canon and Tamron lenses...go figure. I stopped being skeptical of 3rd party lenses. I guess if they want to compete against Canon, they have to make 'em just as good or better at a lower cost.
I'm a pixel-peeping, MTF, intensive sharpness/focus testing, measurebating geek, and this lens satisfies me. Not for tack sharp IQ, but the fact that you have a light carry lens with you all the time, or travel, or when you don't want to grab attention. With a huge zoom range, I can finally work on composition instead of wasting time wondering which lens to mount next.
Anyway, on to the review. "is it sharp?" - yes it is. Is it as sharp as a prime? no...neither is my 24-105f4L when compared to a prime. Sharp sweet spot is 24mm~180mm, f4.5~f10 which is still huge. Color is a little warm compared to Canon which gives a little warmth and feel to the photos. I like... but under Canon's Incandescent white balance indoors it can be a little too orange/yellow. Carry a white card or Expodisc or just use flash. Do I miss IS? yes, but surprisingly this lens with a Rebel XT is so lightweight and short, I don't get a lot of blurring than expected. With the weight of a non IS 70-200 f4 L on a 30D, I get less than 30% hits at 1/focal length rule or slower speed, but with this extra-light setup, my hands shake less and I'm able hand hold 1/80th of a second at 200mm with more than 50% hits!!! Amazing! Tele even at f6.3 can produce nice background bokeh. The focus is fast! Really fast! People that claim that their Canon USM is faster are misleading. Many non USM Canon lenses are louder, slower, and hunts more than this Tamron. I believe the sound makes the lens seem slower than it is.
The verdict: Highly recommend. But, if you're a serious pixel-peeping addict and are looking for top image quality ONLY, and can spend a little more money, then turn to the Tamron 17-50 f2.8 plus Canon 70-300 IS to get the same range. The 18-250 is a great pro-sumer setup and will satisfy most on a budget or need a light carry-everyday setup. I, as a pixel-peeper (shame on me), would not use it for events, weddings or photo-shoots where large prints are needed. I think I'm finally ending my pixel-peeping "sharpness" addiction after trying this lens because the convenience of having 18-250mm at will is incredible! Coupled with a Rebel XT body and a wrist strap, I can stuff it in my jacket pocket and have it with me all the time. It's more than good enough to create excellents photos. Plus, when traveling, thieves won't pay as much attention....it's terrible to have your most expensive gear stolen.