While there may be plenty of good reasons to buy a Canon Digital Rebel XTi, not every camera is good for every photographer. So, depending on what's important to you, and how you take pictures, you should probably be aware of the short comings of this camera. There's plenty of information out there about how great this camera is, but for just a minute or two, lets be brutally and critically honest.
The Canon Digital Rebel XTi, is guilty of underexposure in any and all lighting conditions, specifically with the auto setting turned on. Even Canon will have trouble disputing this claim. I heard from one XTi owner that went as far as to send in pictures taken from two separate XTi cameras to Canon, and ask what was wrong. All the pictures were dark and underexposed, and Canon told them, that yes the camera used to take those pictures was defective. Not realizing of course, that the pictures came from two brand new cameras. Apparently the problem is really with the auto settings. If your the type of photographer, that never uses the out of the box factory settings, then this may not be a issue for you. Otherwise just take a pass, there are plenty of other good cameras you should be considering.
While entry level digital SLR cameras don't always have Live View, advanced models usually do, and they let you compose the photos just like you do with a snapshot camera. The basic models will lock the mirror, and the prism will divert the image towards a sensor that is small. Then, instead of sending it to a capture sensor, it will be sent to the LCD screen, which can take down the performance a bit. If you're getting an older model you might need to focus on the image manually when you are in Live View, though contrast autofocus is available in newer digital cameras.
The XTi manual comes right out and says this is a safety feature intended to protect the flash. Again, you need to judge if this is an issue for you. If your the type that likes to fire off twenty or so pictures in a row, then this camera will likely annoy you to no end. Otherwise, this is really a non issue.
On these digital SLR's, the versatility is limited by the fact that you can't change the default lens. The mirror that bounces the light towards the viewfinder is nonmoving and semi-transparent in most of these models.
With the XTi, Canon made the unusual decision to go the other way. The only way to see what picture your taking, is to use the viewfinder. The 2.5 inch LCD is very nice, you just can't use it to see what picture your currently taking. It's only used to review pictures already taken, or to adjust the camera settings. I guess they're just trying to save battery power. Who knows? Once again, it's all a case of what's important to you and how you take photos.