They're not, the only good thing about them is the unlimited ammo.
I partially disagree - they are extremely powerful, but only if used correctly and if you have HIGH Eridian weapons proficiency (the more you use them - the more it becomes). The Eridian Blaster is the weakest weapon of all of them, to my knowledge and Eridian Cannon is too slow for normal combat (but does amazing damage to stationary targets). But Eridian Lightning (sniper rifle) and Eridian Thunder Storm(shotgun) both rule. At the end of playthrough 1 I would just destroy Crimson lances, Outriders, Guardians and other high level enemies with 2-3 shots of either of those weapons (they also deal Shock damage over time). They require getting used to, but they are worth getting. Also Destroyer is taken down much faster with shots from Eridian Thunder Storm (it's so huge - hard to miss!!!).
Also while the maximum level of all other items in the game is level 48, the max is 49 for Eridian weapons (source - borderlands wiki).