I'm a hardgainer and when I started lifting years ago I used to think and believe that 5-7 reps of heavy weight was ideal for building mass... and then I started to doubt it more and more as time went on when I wasn't gaining anything.
I usually do about at least 80% max weight for everything and do 10-15+ reps. I usually do 4-5 sets for all my exercises. The first set I'll do 12-15, second 12-14, third 12, forth 10-12 and 5th as much as I can, but most likely 10 or 11. I started seeing awesome results after doing higher reps. Think about it.. if you do only 5 or 6 reps that's just about the point where your muscles are warming up, then 7,8,9 you're really pushing it, then 10 to 15 you're forcing more and more bloodflow to your muscles increasing the pump, forcing them to break down (and then re-grow bigger).
Sometimes I will even go to 20 reps if I'm feeling it.
And this is not to be confused with doing much lighter weight because you're doing higher reps. You still need to lift heavy ass weights.