I don't like it.
It's a cool idea, and looks good on paper, but I haven't had it work well in practice. It seems that it can go one of three ways during a single encounter:
- The recharge ability always recharges
- The recharge ability never recharges
- The ability sometimes recharges
That seems fairly obvious, but that's two situations in which the encounter can be significantly different than the DM intends it to be. Recharge abilities are generally fairly powerful; the recharge is a balancing factor. So, if a recharge ability continually recharges, it can be devastating to the party. If it never recharges, then it has very little effect on the encounter, and it's too easy.
It seems to go against the general ease-of-use of 4th Edition monsters. If you're designing an encounter with a monster that has a recharge ability, you don't know the difficulty of that encounter. You expect a dragon to breathe on its enemies, but if it never recharges, then it doesn't have the impact you expected. If it recharges every turn, then you could be looking at a TPK. You simpley don't know, and the game suffers for it.
I don't know how to correct for this. Maybe replace it with a set amount of time for a recharge? So, Recharge 5/6 would become Recharge 5, and the ability would recharge in 5 combat turns. It's not statistically the same, as the former theoretically occurs once every three turns, but that could take into account the fact that a 33% chance doesn't necessarily mean that out of three sample rolls, you get one recharge.
Like I said, I don't know. But recharge makes it difficult to reliably design challenging encounters.