My first DSLR was a Canon Rebel XS, and I eventually upgraded to a T2i. I was in the same boat as you, debating whether or not it was worth spending the extra money for a full-frame camera. Now that I've made the jump to full-frame, do I think it was worth it? In one word: ABSOLUTELY!
For me, the biggest benefit is the wider field of view. I didn't want to invest in an APS-C wide angle lens to get the same field of view my 24-105 now gives me on my 5D Mark II - lenses are expensive - but at the same time I felt limited with not always getting the whole scene in the frame unless I stepped back considerably. Yes, the crop factor on a Rebel helps with the telephoto reach, but it's always easier to crop a photo tighter after the fact than to miss out on a wider shot that includes more in the frame because of the smaller sensor size. I'm not talking about radical wide angle shooting, just normal stuff that you miss out on when you shoot with an APS-C sensor. For example, a group photo of people...you don't always have the luxury of stepping back as much as you want to make sure everyone is in the picture (i.e. indoors, where there are walls or you're shooting in a tight space). I still have a crop-sensor body for some work (7D), but I use it rarely because I love the full frame advantage. The superior low-light performance and increased shallowness of DOF are added perks.
Full frame IS expensive, but in my opinion, it's well worth it.
