I voted +2. The stat bonuses (+2 Dex, +2 Int, +2 Cha) heavily outweigh stat penalties (-2 Con). They get the standard set of elven racial traits, some minor spell-like abilities, 120 ft. darkvision, and spell resistance (which is their biggest bonus, really). And all this is offset with light blindness. So, I stick with +2, but I also think LA buy-off rules (from Unearthed Arcana) should be used. Those extra abilities are signifficant at lower levels, but get less so as the character advances (note that even when the character gets rid of LAs, he's still lagging behind in XP; a LA +1 character can just barely hit level 20 at the same time as his companions).
Also, LA can be situational. For example, a drow wizard fully deserves his +2 LA (he gets his racial abilities, and his +2 Int is a dream-come-true for wizards), while it might be a bit too much for a drow melee fighter or barbarian (racial abilities are nice, but the Con penalty and loss of two points of BAB and two levels of hp seems to outweigh that; drow archers are a different story, though, especially underground (where they can make the best use of their superior darkvision)).
That being said, I think LAs of +3 or higher should be used exclusively with creatures that posess both racial HD and significant racial abilities. For example, LA +3 is (IMO) definitely too high for a svirfneblin.
Regards.