Welcome to the TDP Forums, Randy, and congrats on your new camera and lens!
Seems like you're faced with a difficult decision. In general, any time you look at a superzoom like the EF-S 18-200mm f/3.5-5.6 IS or even the more expensive EF 28-300mm f/3.5-5.6L IS USM (four times the cost!), you're talking about compromise. The biggest downside to such a lens is usually barrel distortion at the wide end - almost every zoom lens has it, but it's more extreme and noticeable on zooms with >5x range. The 18-200mm suffers from >4% barrel distortion; even the L-series 28-300mm superzoom has >4% distortion, although since it's an EF lens, when used with a 1.6x crop body over half of the distortion is eliminated by the sensor crop. Unfortunately, even though the EF-S 15-85mm lens is sharper than the 18-200mm, it suffers almost as badly (3.2%) from barrel distortion at the wide end. For comparison, 'better' lenses like the EF-S 17-55mm f/2.8 IS and the EF 24-105mm f/4L IS exhibit less than 2% barrel distortion at the wide end (and I still notice it in some shots with both lenses).
The other main issue with superzooms is vignetting at the wide end. This is not such a big deal, though, since the T2i can correct this in-camera for JPGs with peripheral illumination correction (and the correction is applied to RAW images if you process them in DPP). Especially if you shoot in JPG, make sure you have PIC enabled and data available for your lenses (done by connecting to your computer and using EOS Utility) - by default, data is only pre-loaded for a few lenses.
Originally Posted by lynx
What you're noticing (and articulating perfectly well) is not merely the result of 'diminishing returns' because of proportional decrease in relative change (18-28mm is a much bigger relative change than 190-200mm). According to Bryan's review, his testing indicates that 200mm on the EF-S 18-200mm is really around 174mm, when compared to two different 200mm prime lenses (the section is the 3rd paragraph down from the starfish picture). So it's a combination of decreased relative change and probably some non-linear decrease in absolute focal length relative to the markings on the barrel, at the long end.
Originally Posted by lynx
I'd do that sooner rather than later. A great way to experience the benefits (and limitations) of a fast prime is the 'nifty-fifty' - the $100 EF 50mm f/1.8 II. Build quality is not great, but it's a nice little lens that will let you experience a fast prime for low cost. For a larger investment, the EF 85mm f/1.8 is one of the best values in the Canon lineup - excellent IQ, fast, and a great focal length for tight portraits on a crop body, for under $400.
The other purchase I'd recommend you consider (do you have a birthday coming up?) is an external flash. If you'll be shooting indoors, you'll quickly discover that f/3.5-5.6 is too slow, and you'll need a flash. IMO, the onboard pop-up flash provides horrible lighting (except for fill flash in daylight). Bouncing the flash off the ceiling will substantially improve the look of your indoor shots. Have a look at the Speedlite 430EX II - I recommend that over the 270EX since the 430EX II swivels as well as tilts, and more importantly offers a red/infrared AF assist lamp (versus the annoying multiple flashes of the main strobe for AF assist).
Originally Posted by lynx
My recommendation would be to stick with the 18-200mm lens. Brendan's suggestion of the 15-85mm + 70-200mm f/4L is a good one, although a 200mm lens without IS can be difficult to handhold. But for a one-lens solution for, "vacations, landscapes, my kid's ballgames, etc.," I think you'll want the flexibility of wide angle to telephoto coverage. Despite the fact that I have an excellent collection of fast zooms and even faster primes covering 10-400mm in total, I still sometimes find myself considering the EF-S 18-200mm as a one-lens solution (in fact, I may take my 7D into a camera shop this weekend and try one out!).
Shoot with the 18-200mm for a while, and that will help you decide where you want to spend your money in the future (a better wide angle like the 17-55mm f/2.8, a better telephoto like the 70-200mm f/4L IS, or something even wider like the EF-S 10-22mm).
--John