This talk about ninjas and robots is all well and good. I have two MM's, one Necro / dark miasma and one Soldiers / traps. Fairly stereotypical in that regard. I play solo a lot, there doesn't seem to be that many around when I play and besides, I don't mind that, I can pick my own pace.
I find that the play experience for the two is very different, which might be surprising for two of the same class. Both are mid-teens, so my experience isn't that extensive, but here goes.
Red Guard - Soldier/traps. I use the attack powers a great deal, since everything else is used in preparation. Before larger battles I tend to drop triage beacon, caltrops in front, then me and the goon squad opens fire. Triage isn't impressive, in fact rather depressive, but it seems juust good enough to keep the soldiers in good health in a long battle. Unlike the zombies, the soldiers all use ranged by preference, so this setup works fine - the caltrops delay the melee hitters a little bit, and they are pretty good at taking down people, but constantly need to be told new targets since their attack range is much greater than their aggro range. The soldiers seem to be able to soak up a bit of damage, but oddly enough they do not seem to gain a new attack by equipping them. The specforce guy is a godsend. FLashbangs with area disorient is his one great power, otherwise better range, more damaging bursts and the occassional grenade doesn't hurt (me) either. Very methodical advance through the missions. I am not too impressed with caltrops, it seems like a weak version of tar patch, and the web grenade is also kind of useless. Triage seems to work, but it is slow and leads to very 'set piece' fights.
Soulcatcher - Necro/dark miasma. The zombies are very superior to the soldiers in damage dealing - part of why I wonder if soldiers lack an extra attack. They use a sort of cone barf acid attack thing before each combat, with area damage, The grave knight is also damn nice. In an undead sort of way. WHich is to say not nice at all. This is not at all as methodical and passive as with Red Guard. I hardly ever use the attack power, I don't plan to get one beyond gloom. Not because gloom is good, but because I hardly use that, I don't need another I won't use. I spend my time healing, tar patch, darkest night, and otherwise pop the guys back up because the zombies die a lot compared to the soldiers, but lieutenants go down in seconds to the undead horde. Soulcatcher is by far the most team friendly of the two - all her powers are generally useful ones that benefit any team, including teleport / recall friend.
Red Guard on the other hand has nothing to contribute to a team except firepower, at least until the FF drone.
Mastermind is a very fun class to play, that's the one thing I'm sure I won't regret.. I wouldn't care to try my hand at PvP though, it's bound to be a frustrating experience.