The T3i has essentially the same body as its predecessor, the T2i, but with an articulated instead of fixed LCD.
With a few exceptions, the T3i's body and interface are almost identical to the T2i's. It's slightly heavier (but not larger) thanks to the articulating LCD. It feels sturdy, and though the textured, rubberized grip feels kind of cheap, the camera is comfortable to hold and shoot single-handed, and can stand up to the weight of a good lens.
Canon carries over the control layout and user interface from the T2i, although it has moved the Display button to the top and replaced it with Info. Camera operation is straightforward. On the back there are direct-access controls for Live View/video recording, exposure compensation, white balance, autofocus method (single, AI focus and AI servo), drive mode, Picture Style, AE/AF lock and focus area (single-point manual or all-points auto). You can also change these settings, plus metering, flash, image quality, and a few others, via the typical Quick Control screen.
My one quibble here is that the buttons all feel a bit too flat.
The mode dial includes the usual set of manual, semimanual, automatic, and scene modes. It
doesn't lock, which may bother some folks (though not me). As with the 60D, however, I find the placement of the movie mode--at the opposite side of the dial from the advanced modes--insanely frustrating. I've actually missed video opportunities by having to scroll around from shutter-priority mode to video. Ironically, this design is more suited to pros who plan to use the camera as a cheap video dSLR and never take it off the video setting than to the consumers at whom it's ostensibly targeted.