There's no less than 10 kinds of mephit in the MM alone, because adding lots of low-level extraplanars apparently had a higher priority than writing comprehensible abilities or fixing the CR of celestials. Yay. On account of their high number, I'll be covering them in parts.
All mephits are outsiders with 3 RHD, putting them somewhere near the imp and quasit in terms of chassis. I'll be comparing them to these fiends where needed.
Currently, all mephits have a cohort LA of +3. I don't expect any to stay there.
Ability scores are good, overall, with a large bonus to dexterity and a smaller one to charisma. Their airheadedness is annoying, and sadly common to all mephits, but a 60 ft. perfect fly speed helps make up for that, as does DR 5/magic.
Their breath weapon is poor, though its relatively short recharge time and possible feat support mean it has at least some potential. 1/hour Blur is actually quite good for a SLA, and it doesn't decrease in power much at higher levels. However, Gust of Wind 1/day is forgettable.
Finally, there's their fast healing, which triggers incredibly easily. As long as there's air around, an air mephit can just fan itself to heal, which may not even require an action (ask your DM!).
IMO, all this places air mephits above equally-leveled characters, and adjustment is in order. +1 LA.
Dust mephits are similar to their airy cousins, except their flight is a bit slower, their breath weapon is smaller and sacrifices some damage for a debuff (worth it, IMO), and their Gust of Wind SLA gets replaced by a high-CL Wind Wall (a very welcome trade-off). Sadly, their fast healing only works in dry and dusty areas, making its activation much harder.
All in all, it seems to me like these are about on par with air mephits, and therefore deserve the same LA. +1 for dust mephits as well.
Together with gargoyles, these mephits are amongst the few earth creatures with fly speeds. Their stats are very nice for a melee character, especially with their 1/hour Enlarge Person (self only) SLA. Their other SLA, Soften Earth And Stone, has its uses, though it's very environment-dependent.
The breath weapon is identical to the air mephit's (that is, boring), but at least their fast healing should be reasonably easy to trigger.