I use a Thunder B CO2 grenade basically because I bought some cheap in the mistaken belief that they would be allowed at the National Airsoft Event at Ground Zero. They are not

- the best thing I can say about them is that once the enemy know what they do, ie how loud they are and how much they jump when they go off, they create a definite intimidation factor, but they are very unpredictable in how long it takes them to go off from the moment of spoon release - not that you need to worry about them going off in your hand, but that they can take ages, half a minute or even longer...
I've also recently bought an Oshiboom impact BFG which i've used effectively at a woodland site for approaching enemy cover under suppressing fire and dropping over/around it.
I would never be without lit stick bangs though, just because you can throw them a lot further: ideally MK9's just because they are big enough to throw properly, but MK5's will do. The thing to remember about them is that the strikers can be quite unreliable, so keeping half the cover of a box of Cook's Matches handy is the way forward, or one/some of our own NH Shooter's MK5 flashbang holder/strikers.
To prevent loss of expensive reusable grenades I attach key finder fobs to them with tape.