Good points : If your brake goes shit mid ride or you need that extra bit of hold its always an option.
Bad points : If it rains your brake will become non existant. It makes a bloody horrible sound compared to a setup with just a grind. It looks horrible in my opinion but that doesnt really matter. Too much of it will make your brake pads stick to your rim and not release. I found that when you use it alot, like i used to, as you keep refreshing it, you get 'dead' tar which stays on your rim but doesn't actually do anything.
To be perfectly honest, you dont really need it. If you spend a bit of money on some decent brakes pads, for example Heatsink, Tnn, Coust ect then it will be better. Get a decent rim, good sharp grind and a decent bleed on your brake and your away!