I bunned for 4 or 5 months and my hair grew, and grew and grew! I didn't take measurements, but I can say that it was significant. I mean really significant. I feel as if my hair got thicker as well... I did Southern Tease buns and high buns and my hair loved it...but I didn't, lol.
If you want longer, thicker hair, buns are the way to go...but for me it's not worth the sacrifice. I'm retaining length now (with my hair down everyday), but I think my hair would be even longer if I was still protective styling.