I play mostly new games, but when I beat or get bored of those I often go back to older games. For example, I started a new character on KOTOR yesterday because I want to take a break from Halo: Reach and have beat the rest of my RPGs.
What I like to do is to play several short under 10 hour games, then get into a 50+ rpg. I just replayed Final Fantasy 9 on my PS1 and finished it at 60 hours. That game is extremely beautiful for a 2 gen old game.
I've got mostly all of the past systems (NES, SNES, GENESIS, PS1/2, N64, etc.), and tons of games on each. While I tend to stick to the modern stuff on the 360, I still find myself dusting off one of the old consoles and popping in a classic every now and then.
Hey Trashcan Man! What did old lady Semple say when you burned her pension check? Boston Bruins - 2011 Stanley Cup Champs!