I went through this myself looking for a single lens to take on hiking trips. The canon 18-200 was a clear winner, but be warned that sharpness from any super zoom is nowhere near a great zoom like the 17-55 or any of the 70-200's.
It is also the worst lens that I have ever used for "lens creep". Mine has a rubber band from a head of broccoli as a permanent feature to keep it under control. Also, this lens does NOT play well with teleconverters.
I made the same choice a few years ago, thinking I could live with a little lower IQ and take more pictures since I wouldn't have to swap lenses.
Now, my 18-200 pretty much sits in the bag. It's not a bad lens when you take into account its range, and I have used it to get some nice shots. It just doesn't hold up to my other lenses. I've gone back to shooting with primes for the most part, the same way I shot back in the FD days.
It is convenient to have the range, and I haven't gotten rid of it. I just don't use it as much as I thought I would. It might be something to borrow/rent (if that's possible) to get a feel for it before buying. If I had to go back, I think I would have put the money toward a 200mm 2.8 L and stayed with my kit lenses for zooms.