I almost bought XTi before I got my K100d yestaurday, since I only got it yestaurday, I couldn't say much about the performance~
However, I did a lot of research b4 I made up my mind. Main weakness for me as a SLR noob of K100d compare to XTi are 1. six megaplxels comare to 10, 2. Small buffer...
Main advantages are 1. much cheaper, 2. quality kit lens, 3. it feels better in my hand...
K100d is my first SLR, but I've been shooting with digital camera for at least 6~7 years. I have never print anything bigger than 5x7, therefore 10 megapixels are not that important factor to me (another main reason I didn't consider K10d) Also I can get two quality lens for cheap (18-55 and 50-200), that's perfect for a noob like me to practice on...
For same price, you can get 2 lenses with K100d, if you have never touched DSLR before and you don't really need that 10 megapixels. I would recommand K100d over XTi or K10d, just my .02~
Anyways, so far so good for me and I am just a beginner... good luck...