I've been playing didder for 4 years now. Started on Max home, then I moved out to playing in arcades and such. I can AA almost all 8 footers, and I'm working on that same accomplishment with 9's. Just don't have the time to be heading out to arcades to do that nowadays... plus with all of the SN machines popping up, I'm losing interest.
I just started playing ITG last May. I'm only decent though, the best I can do right now is getting a double star in Summer Speedy. Out of both franchises I'd prefer ITG solely because of the interface, harder songs, and more catchy tunes. Oh and R21, lol.
Tips... hmm... play with ALL of your foot [meaning don't keep your heel in the air when your stepping] and don't lift your feet entirely in the air [like you're jumping] between every step. You'll get really tired like that. To raise stamina for the more harder songs, I've found that daily bike riding helps a lot.