A member of my CL club has been using a predecessor of E-510, the E-500. This past season, we had an opportunity to shoot similar events. His results were comparable to my K100 and even better(he's a MUCH better photographer). We've swapped cameras just to get a feel for the differences. Pentax is heavier, e-500 is quicker to focus. In the end we both came to the conclusion that BOTH cameras are comparable. Unless you plan to make a living by selling great photos, either camera will be just fine. As for XTi, build quality is indeed questionable but is adequate. I owned an XTi for a few weeks before exchanging it for a K100D.
Pick E-510 or K100D and you will behappy with either choice. XTi is a second tier contender.
