I am generally not a fan of the Nerf Melee weapons. In a NIC setting, you will get shot by a competent player long before you ever get into swinging range. In our wars, we allow melee to be thrown, but most of the foam melee is not weighted to be thrown well. With the exception of carrying something for stylistic barrel taps, melee doesn't work well in the primary rounds. However, there is one melee weapon I've found a use for; the Nerf Thor Hammer:
The hammer isn't very large, which ends up being an advantage when you carry it as a secondary. Its head is completely made of foam, so it's weighted well enough on one end to be thrown reliably. With a strong toss, the Nerf Thor Hammer will easily go 70-80'. Plus, it provides something different to break up the rounds with. It's just a lot of fun.
If you want a cheap melee weapon to play with, the Nerf Thor Hammer is a good choice at $9.99. It's hardy construction allows it to last through the abuse of being thrown cross-field for several uses. And there's nothing like getting a kill with a foam hammer while screaming, "I am Thor, God of Thunder!"