I used the machine gun through most of the game, and prefer it over even the particle cannon. I find that starting out, 3 snipers, 3 machine gunners and 2 grenade launcher/rocket guys are a good mix. The only problem is that later in the game, the laser rifle is really good, but it requires assault skill and not heavy weapons. The heavy laser isn't nearly as good, and neither is the particle cannon due to high TU costs and low accuracy. The machine gun is fantastic in covering a team on reaction fire, as well.