Imagine a mayhem-minded toddler, except it's also on fire and spreading the fire to everything it touches, and you got the basic idea of magmins. Dwarf Fortress players may imagine a horde of fire imps instead.
Magmins have two elemental RHD, which are inferior to aristocrat hit dice but do come with a few welcome immunities. Just try not to die: True Resurrections are hard to come by. DR 5/magic and natural armor should hopefully help a bit here.
The little elementals have orc-level strength despite their small size. Their constitution score has a small bonus, their intelligence a penalty. Their natural weapons are a weak slam and a slightly less weak flaming touch, but both are made a bit more interesting by their Combustion ability: the 1d8 damage is apparently taken again each round for three to six rounds.
A magmin's Fiery Aura is more of a drawback than an advantage. It can't be turned off and requires people to stay 20 feet away from you or get burned. Especially in narrow dungeon corridors, this may cause trouble.
Finally, there's Melt Weapons, which apart from being nonsensical (how is something hot enough to melt adamantine but not to scorch wood?) is a very good way to destroy loot. Again, it can't be turned off.
All things considered, I think magmins just aren't that good an option. Various abilities of theirs are harmful to the party as a whole, their offensive traits just aren't that good, and the elemental type's immunities are obtainable in other ways as well. +0, with serious consideration given to -0.