While the new 18-55mm IS II kit lens seems to display a little more fringing than its predecessor, the T3i itself has rock-solid photo quality, even at moderately high ISO sensitivities.
Overall, the T3i has an excellent noise profile, unsurprisingly similar to that of the 60D's. JPEGs look very clean up through ISO 400, and even at ISO 800 you have to really scrutinize to see the beginnings of detail degradation; at ISO 1600 the noise becomes more obvious but still isn't too bad.
ISO 400 is sort of my tipping-point sensitivity; to shoot action outdoors, I generally have to bump up the setting to at least ISO 400 in order to reach a sufficient shutter speed. And because few consumer cameras are fast enough at shooting burst raw+JPEG, the in-camera JPEG processing has to be decent as well. The T3i fared pretty well under these conditions. There's some color noise in the brown fur and some mushiness in the details, but both are typical in this class. Overall, I consider this good enough to use, but still wish I would have been able to shoot raw to clean it up.
(1/125 sec, f7.1, spot meter, AWB, ISO 400, 18-55mm IS II lens at 55mm)
Tiger closes her eyes when I use flash, so I bumped the T3i up to ISO 800 and hoped for the best--and the results were pretty good. But you can also see here how shooting raw (bottom) allowed me to clean up the distracting color noise that in-camera JPEG processing never seems to clean out of brown Tabby fur.
(1/100 sec, f4, spot metering, AWB, ISO 800, 18-55mm IS II lens at 24mm)
Overall, Canon's JPEG processing remains very good. Even at ISO 1600, I couldn't obtain unambiguously better results processing the raw--Canon seems to optimize for exposure at the expense of sharpness, and I couldn't get sharper results without losing some shadow detail (you may do better).
(1/30 sec, f3.5, spot metering, AWB, ISO 1600, 18-55mm IS II at 18mm)
The 18-55mm kit lens displays visible but not terrible asymmetric barrel distortion at its widest. In shots with the previous version of the lens, the distortion looks more symmetrical, but I don't have exact comparison shots--they put up scaffolding months ago which prevents me from replicating the shot.
(1/100 sec, f5.6, spot metering, AWB, ISO 200, 18-55mm IS II at 18mm)
On its Standard (default) Picture Style, the T3i plus 18-55mm IS II lens can produce some nice sharp results. However, notice the fringing on the highlights and processed-looking mushiness on the out-of-focus areas; those are my two least-favorite characteristics of the new lens.
(1/30 sec, f4.5, spot metering, AWB, ISO 200, 18-55mm IS II at 36mm)
The new lens seems to have worse fringing problems than its predecessor--not just on the high-contrast edges on the sides of the frame (top call-out), where we expect them to be, but real aberration issues on internal areas (bottom call-out).
(1/60 sec, f4, spot metering, AWB, ISO 200, 18-55mm IS II at 18mm)