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!
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.
The 5d3 is my third camera and I just love it. It is expensive but the rewards are worth it. I was using my 7d for bird shots but now have almost stopped using it since the 5d3 can get the same shot with a sharper image with cropping. The auto focus is great as well.