It really depends on the army for me. If I'm using the BA, I stay away from any rapid fire weaponry of any type, so plasma is out. However, I use plasma extensively in my IG army, usually with a plasma gun in every infantry squad I run.
The way I see it, it has a longer overall range than melta, so if I include a heavy weapon in the squad it allows it more turns of shooting before the enemy gets to me than a melta would. If I don't include a hvy wpn, then the melta still isn't a great choice, as the rest of the squad has rapid fire weapons as well, so if any of them shoot I don't get a charge anyway.