The T1i is a very excellent overall camera. On Canon's level, it lines up slightly above the entry level category, but in my mind, not up into the semi-pro cameras.
My only real complaint about this camera is, it's like holding a toy. Confidence is a big part of taking pictures, which you can gain part of that from how the camera feels in your hand. That being said, I'm over six foot. . .with slightly bigger hands. So it might be all right for some of you.
If you are looking at the XSi, don't get it, get this if you can spend the money. Most people say the increased resolution on the camera isn't worth the price. But that's only one aspect of the increase.
This things runs way faster than the XSi, so if you don't care about speed, that's fine. Don't underrate the importance of the new LCD screen, it's way beautiful, and you might have a chance of seeing how clear your images will be. Which also attributes to the intuitive color changing menu.
AND REALIZE, if you ever wanted to use L glass with this bad boy, you can and it will produce beautiful images. This would not be possible with the XSi.
Overall, I am very satisfied with this camera. I have produces some excellent images!