Well, I've got the Pyromat chair (shown on the firebox link from that page), it was okay until my brother (who is considerably heavier them my 9 stone self) sat on it. The seat sagged into the chair and I can feel 2 blocks of wood digging into my legs (so I guess a cushion would solve that).
tbh, the biggest disadvantage with these gaming chairs is needing somewhere to keep them out of the way when they're not being used.