I have the Tamron 18-200 for now, but I'm in the process of upgrade lenses since I'm not very happy with it.
It's a nice lens and you don't have to change lenses all the time, but it has several problems:
1. Resolution - I found that the picture quality is not great, mainly on distant objects, when the object is close the quality is not bad at all.
2. Focus speed - it takes quite a lot of time to focus, the motor is slow and the lens makes it harder for the camera to focus.
3. Focusing under low light - the camera sometimes can't focus on low light (and it's not the camera problem, with my new 17-40L it focuses amazingly fast).
BUT, the Tamron 18-250 got much better reviews, so it is probably better. I decided to go for 2 Canon lenses instead of my 18-200, but it's my decision. I guess others can be quite happy with the 18-250.
You can read about both in photozone:
Canon EOS 50D, Canon EF 17-40L, Canon EF 70-200L f/4 IS and Canon SpeedLite 430EX