I feel like this is relevant here.
Nymphs have 6 Fey HD, which are interesting for giving good saves and skills but bad everything else. They're a bit like Beguiler hit dice, but with better reflexes and worse proficiencies. Their chassis is pretty good: a net +28 to ability scores, DR 10/cold iron, and a swim speed (because why not?).
In addition to this, nymphs get various special abilities. Dimension Door 1/day is good for being standard-action mid-range repositioning of most of the party, and Stunning Glance is a save-or-lose with a good duration.
Unearthly Grace is good for reasons I shouldn't have to name: I've seen characters that spend two levels on getting a worse version of it. Blinding Beauty is somewhat less great: while humanoid opponents will probably still appear from time to time, activating the ability means any nearby humanoid allies of yours get exposed as well.
Finally the centerpiece of any nymph-using build: the druid casting. Your typical 6 HD nymph casts as a 7th-level druid, meaning a LA is most definitely in order here.
However, compared to druids nymphs are worse off in a number of ways. They lack an animal companion, and the same goes for Wild Shape. That said, a LA of +3 is still required to keep them balanced.
Next are ogres.