I recently went from a T2i (which I loved btw and kept as a backup) to a 5D Mk3. The differences are amazing, but not entirely in the way that "hey buy a $3500 camera and your photos will immediately get better". I upgraded to FF because of the better ISO performance, the 100% view finder, more AF points, weather sealing, etc. My T2i did not have these features and I got the Mk3 on that Adorama eBay deal 2 weeks ago. Anyway..
The color reproduction and clarity of the Mk3's sensor and 24MP compared to my T2i's crop and 18MP sensor are very noticeable to me and some other photo buddies. However, if you feel you're missing that extra punch, that could just mean more practice in composition and lighting and such. People do amazing things with, and I hate to say it, cell phone cameras. That punch comes from subject, composition, lighting, creativity. Sure having more resolution and better color reproduction and in some cases high ISO ability would come in handy.
Don't get me wrong, its a great camera, but I still take crappy shots with it. =)