It's kind of funny that the erinyes' description explicitly says they appear as both genders, yet there's only a handful of images depicting male erinyes. Even WotC themselves don't seem to have made any.

Erinyes aren't very different from your standard Baatezu. Average land speed, good fly speed, natural armor, energy immunities... it's all been seen before. Medium size is a pity on a creature that's going to use weapons, but your stats (both physically and mentally) somewhat make up for it.

Spell-likes are decent. Charm Monster at-will is good in social situations, Greater Teleport is expected, and Unholy Blight can function both as a mass debuffer or a minion sweep. Minor Image is quite underwhelming, though.

An erinyes's summoning ability is interesting in that it doesn't allow the devil to summon a being of its own kind: only the weaker lemures and bearded devils. At least your chances are better than the usual 35%.

Then there's permanent True Seeing (very nice) and Entangle. Sadly, the latter is not as good as the spell of its name. Spending an action on something that's barely stronger than a cheap alchemical item isn't something one should do at this level.

Erinyes also get Dodge and Mobility as bonus feats, which will mostly be useful for prerequisites. Elusive Target is a notable feat that gets very attractive without the feat tax.

So, what's the final LA? There aren't any fiends in the ECL 9-10 ratio to compare them with, but I'd give erinyes +1 LA. They lack the large size and superior spell-likes other fiends start to get at this level, but are nonetheless too strong for +0 LA.