I stepped from the XT to the XTi to the T1i and now have the T3i. I can say with confidence that my T3i is 10x the camera that my XT was. More MP, better IQ, better ISO performance (though not 5D3 level, for sure), bigger/better LCD, more ergonomic, and better menu system. I mean, it's a whole new camera and well worth the upgrade!
The best thing is that it's still light and compact. I've carried it all over the world (camping) and have never had an issue with its lack of weather sealing or build-quality. The Rebel series is much maligned by purists in search of the ultimate IQ (don't we all fit into that category on some level?), but they fail to appreciate its advantages: size matters.