Delvers are another one of those monsters with literally no use for any DM, ever. They're docile and don't attack people and have no abilities that makes them worth seeking out. There's the possibility of a metal-drunk (check the description) delver terrorizing people, but if you're level 9 and your quest is: 'kill the giant earthworm-thing that got drunk on metal', you should find a new quest.
With that said: how does the delver stack up as a PC? Not that great, which is largely because of the fifteen aberration HD you have to take.
The ability scores are good (not difficult to attain before this level, though), huge size is a plus and any adventurer can use tremorsense. The delver's special abilities are moderately useful, with their utility increasing if you're fighting earth elementals in an underground environment or something? I honestly don't know what fifteenth-level adventurer can't just get someone to cast Stone Shape for him.
Add to the HD and lackluster traits an inability to wear armor or wield weapons, and -0 LA is the only possible answer to: "What LA should a delver have?".