My Canon T1i arrived Thursday. We were putting our new lawn in this weekend so I haven't had much of a chance to play with it yet (except to take some photos of our progress) but here are my first impressions.
The camera feels lighter than I expected. I guess I am thinking back to the days of my old Minolta Maxxum 5000i (body selling for $30!). I am impressed by how light it feels.
The next thing I noticed is how large and bright the LCD screen on the back is. It is at least three times the area of my Canon S3. It sure makes chimping much easier. My wife was very disappointed to learn that you don't get a live preview of the shot on the LCD, though. She's used to that from the S3, together with the flip-out LCD. Time will tell if that bothers me. I don't think it will, because I typically shoot looking through the viewfinder anyway.
Taking a picture in full auto mode is about as easy as you might think. The nine focus points are clear in the viewfinder and it is very helpful to know what point(s) the camera wants to use for focus. The shutter release is smooth and it feels like a serious camera. I don't know how to explain it better than that.
The autofocus is noticeably faster than the S3, especially in low light. The T1i doesn't waste time hunting for focus; it seems almost instant. I pointed it at a stippled white ceiling in low light, and the T1i had no trouble focusing on that. The S3 would have hunted for focus and given up in the same situation.
I haven't tried the video at all. For now I'm going to stick to stills. The T1i is capable of "full" 1080p video (1920x1080 pixels) but only at 20 frames per second. I think for train shooting that is going to be insufficient, so I will likely use 720p video (1280x720) at 30fps. The S3 shot at 640x480 so this is still a big improvement.
Those are my first impressions. I have a lot of training to do to get used to the camera and its features and drawbacks. But it's a start.