Extreme range zoom for 60D

I have a 60D. My two main lenses I use are my 17-55IS and 70-300IS. Both Canon. I am extremely happy with the sharpness and general image quality of both lenses. Sometimes it's a bit much to bring both lenses and change on the fly.

I know some of the more extreme range zooms compromise image quailty in a big way. None seem to be highly rated. The Canon 18-200 seems to be a huge compromise on quality. Tamron has an 18-270 that actually seems to have decent ratings and opinions. Anyone have experience with these lenses? Opinions? Share some pictures taken with them?

I even looked at the 18-135 IS. Although it's not an extreme range zoom it's got that extra reach over my 17-55 IS.

I'm basically looking for something I can take on vacation or certain situations where lugging lenses and changing is not practical.

Any thoughts and opinions are welcome.

Thanks,
Keith