I did the lvl 70 heroics when I first got my team to 70. My paladin tank (pre boxing) was already relatively geared, and I wanted the challenge. I got decent exp out of it, got to experience the content I missed out on due to fail pugs, and such, I learned a lot for heroics in general, and was able to apply what I learned to lvl 80 heroics.

We have one boxer that boxes just because he wants to see old content, that isn't easily soloable. He regularly 5 mans the lvl 60 raids, and on more than one occasion we've gonoe and done MC, and AQ as a guild. One time in AQ, forget if it was 20 or 40, but we were just gathering tons of mobs, and AOEing them down (i was running 3 mages, a holy priest and a prot pally, so I had AoE galore) and we didnt even notice that a boss was part of the pull until we looted the epix.... that was a lot of fun