Well, IMO, the 37mm launcher is a pretty nifty little toy.
I say that because, unless you're LEO, you're going to have a difficult time finding any ammo capable of real use. An argument could be made of parachute flares, dragon's breath, etc., I guess. But I can buy a lot of hand-held parachute flares for the cost of a launcher.
Even if you can get the Destructive Device stamp for the 37mm or 40mm, you're still going to be paying out the wazoo for "useful" ammo. Unless you can find a military or police source that will sell to you, you're going to be stuck with rubber ball, baton and, maybe, OC/CS rounds along with the better smoke rounds.
Now, this doesn't mean to say that you cannot "roll your own," as a lot of other 37mm guys do. But unless you know what you're doing, anything you can generally whip up is going to be comparably anemic.
That doesn't mean you still can't have a lot of fun with one, just that you shouldn't expect anything near the level of results with which you may be familiar with military 40mm ammo.
I'm sure you may also find some form of slight tactical advantage with the use of some of these, as well. (Emphasis on slight.)
Hope it helps.