Ive just upgraded my 550D to a 70D for land use (Keeping the 550D for underwater until i can find £2500 spare for a new housing...). I've noticed a big difference. Firstly, the viewfinder, bigger, brighter - much easier to frame the shots and read. The electronic level is great as well.
The AF system - with the same lenses the 70D focuses much faster and much more accurately. The AI Servo tracking modes are a lot more comfortable and a lot more accurate.
Faster burst is good for action stuff and im getting a bit less noise at a given ISO.
Ergonomically having a dial and a wheel makes adjusting things so much nicer (although there is no fast white balace set button now which im missing).
Certainly for me a worthwhile upgrade, although there was noting wrong with the 550D the 70D is allowing me to get a better success rate and more chance of getting the images i want due mainly to the viewfinder and better AF performance.
Hopefully magic lantern will appear on it at some point.