Jowgen has already gone into most of what these balls of holy light can do in detail, but his most important points I'll repeat here.
Lantern Archons are 1 HD celestials with various interesting abilities, including (but not limited to) the creation of everlasting magical fire, the granting of temporary HP, flight, the capacity to shoot rays of searing light, and the ability to teleport to any point on the same plane with perfect accuracy at-will as a standard action.
Not bad at all. The lack of positive stat boosts (combined with the strength and intelligence penalties) hurt, but still. I actually find all these abilities worth +3 LA, making a lantern archon an ECL 4 creature.
Now, this may seem like a lot, and I can already imagine some people worrying about your HP. Rest assured, there's nothing wrong with your survivability. Imagine a lantern archon with 13 constitution. It'll have only 8+1 HP, but it will also be
walkingfloating around with 11 temporary HP most of the time, so it might be more accurate to consider it as 20 HP. A 4th-level rogue with 13 constitution will have 6+1+3.5x3+3=20 HP as well, and the rogue lacks damage reduction, protective auras, or flight.
So defensively you won't fall short. How about offensively? A 4th-level warlock can deal 2d6 points of damage with Eldritch Blast, a lantern archon can make two light ray attacks that deal 1d6 points of damage each. The warlock can apply various invocations, but the barebones ability is about equal. A similar case would be a caster with Acidic Splatter, also capable of dealing 2d6 points of damage at-will.
Finally, will the lantern archon be able to keep up as the party increases in levels? Most likely it will. Rogue, or a similar precision-based class will benefit from the two free natural attacks (remember, low strength doesn't necessarily prevent you from wielding weapons), and a caster will benefit from the flight and can still reach 8th-level spells (or 9th-level spells with LA buyoff).
Final verdict: +3 LA.