I went through the same thing last January.  I already owned the Canon EF-S 18-55 (non IS that came with my EOS Digital Rebel XTi) and the Canon EF 55-200 USM II.

I was going on a trip to Costa Rica and wanted to have a better telephoto lens and if possible cut down on the equipment by having a single lense.  After getting some advise from other photographers I know, I decided to go with the Canon EF-S 18-200 IS.

Although, to this day, I am still unsure if I made the right choice. This lenses has a very pronounced corner effect even with the correction applied by my T2i casing. Also, the lense is wider with 70mm threads, this means I had to buy new filters and would have to buy a new set of cokin filters to upgrade from the A to the P series.

Looking back, I may simply have been better off getting the EF-S 55-250 IS instead to replace the 55-200 I already owned.