My group has played through Act I and are part way through Act II. We have 3 players, 2 heroes and 1 overlord.
The one big issue that we have is that the monsters are pretty much immortal. There is very little point in actually killing a monster. In most quest, every time you kill one, another of the same monster just shows up and attacks you again.
Are we misinterpreting the reinforcement rules? Assuming we are playing it right, it makes one of the key aspects of the game, killing stuff, very unsatisfying.