Caleth's answer is good and correct, both in general and in this specific case. I'm going to dive into the abilities a bit more than they do.
To be clear and complete: as Caleth said, Savage Species is 3.0; 3.5 increased the HD of an erineys to 9, so a starting erinyes PC would be ECL 16 without any class levels.
Erinyes have a couple of things going for them:
- Fly speed: always useful; having it naturally means not having to spend money or spell slots on flying. But, at 30th level, popping overland flight once a day shouldn't be a big deal
- DR is always a nice perk; traditionally, DR/good is uncommon to be bypassed by foes (IME, though the fact that a devil is a plausible character may change that assumption).
- Darkvision: nifty, but trivial to gain through other means.
- Immunity to fire and poison, resistance to acid and cold: nifty, but trivial to gain through other means.
- See in Darkness: highly situational, but darn nifty when it comes up.
- Telepathy: nifty, but trivial to gain through other means.
- True Seeing: this will always be good to have. It's not hard to gain through other means, but it can become costly to do.
- Greater Teleport (self + 50 lbs of objects only) at will: this will always be useful. Sure, it's a standard action, but being able to go anywhere on the plane at any time is solid battlefield control and fantastic for picking up a scroll for the wizard or selling off loot.
- Outsiders don't need to eat or sleep, though they do need to breathe. Which, that makes them a solid choice for standing watch and means they don't want a Ring of Sustenance.
- +10 Str, Dex, Con, Cha; +8 Wis; +4 Int. Not bad, especially for a martial character.
They also have abilities that look fantastic, but aren't.
- Entangle and their other spell-like abilities are worthless: the DCs are laughable, even if the devil manages - somehow - to bypass the target's SR.
- Summoning other devils is a neat trick, but the devils it can summon are only useful for trap fodder (and, they can only be summoned half the time).
- The erinyes' own SR is laughable at ECL 16, let alone ECL 30.
- Dodge and Mobility as bonus feats: better than nothing; neither feat is terrific on its own, but having them opens up Spring Attack which can be quite useful in the right circumstances (eg., positioning so that the rogue can get flanking more easily). It's a highly situational feat, to be sure. In the specific case of the erinyes, though, Flyby Attack is categorically better (as long as the GM isn't nerfing non-magical flight by putting hard limits on where/when they can fly). HT to @Hey I Can Chan for reminding me of this feat.
Being an outsider is a double-edged sword: there are a bunch of spells that simply don't work on outsiders, but (a) most of those are very low-level (eg., Charm Person) and unlikely to come up at level 30, and (b) spells that don't affect outsiders include Resurrection (True Res works, of course). Outsiders don't have the body/soul duality that most creatures have: their body is their soul made manifest (where a human's soul is attached to a mass of flesh and bone), which can make things weird (mostly at the GM's discretion).
So, an erineys gets some nifty abilities, including a couple that are decidedly non-trivial to gain other ways (especially See In Darkness and True Seeing). But, they're giving up 7 levels (via LA) of BAB, feats, saves, feats, class abilities, feats, skill points, feats, HD-based stat bonuses, and feats to get them. The stat bonuses help offset that some, but even at +10 STR is only a +5 to hit where taking 7 levels of a full-BAB class would grant a +7.
On the plus side, "outsider" is one of the stronger racial hit dice to gain: d8 HD isn't bad, full BAB, all good saves, and 8 skill points per HD (with the standard x4 at 1st HD).
IME, the vast majority of monstrous creatures fall into this trap: there are nifty features that look great on paper, but the don't scale at all. Even one or two racial HD or LA can be extremely painful, which is why there are optional rules for buying off level adjustment - the game realizes that
When a character with a level adjustment advances in experience, the level adjustment he started with becomes more and more of a burden. Eventually, the benefits of the creature type may come to be eclipsed by those of his class features, and the player may regret his choice of race.
What are the implications of having an erinyes character alongside non monstrous characters?
There's another dimension to this question: how will the rest of the party react to a literal devil in their midst, and how will NPCs react? In virtually every campaign I've been a part of, the answer is "not well", ranging from the devil needing to go to fair lengths to hide their nature to needing to avoid the "torches and pitchforks" brigade.
Also, remember: devils are literally made of evil and law. It's not that they choose to be evil (which, in the D&D sense, means treating others as just a means to an end), it's that they're quite literally made of it. Depending on the GM, it might be that they have incredible difficulty in being non-evil (or non-lawful) - requiring intentionally choosing to act against their core being - or that they can't choose to be good (or chaotic!) any more than I could choose to be a silicon-based life form or flap my arms and fly to the moon. In the latter case, the extent to which this matters is heavily dependent on the GM, and on how much buy in there is for "working with this party serves my greater interests; I'm not so stupid as to backstab them for funzies, at least until my goals are reached". This could set the stage for some very interesting role-playing, or a very frustrated character who needs to sit outside in a forest yet again while the rest of the party gets to go sleep in the inn.
The former is RAW in the MM, p305 (ht @KRyan):
Always: The creature is born with the indicated alignment. The creature may have a hereditary predisposition to the alignment or come from a plane that predetermines it. It is possible for individuals to change alignment, but such individuals are either unique or rare exceptions.
... but, this GM has always leaned heavily towards the latter for outsiders with alignment-based subtypes (barring divine intervention, which I suppose isn't all that hard at 30th level). At the very least, this GM would be disinclined to hand-wave an outsider with alignment subtypes trivially becoming a different alignment. I've had good experiences with this interpretation of the nature of outsiders at my tables, leading to interesting RP questions about how to deal with devils (and, to a lesser extent, demons) that aren't immediately hostile. In my current Pathfinder campaign, a cleric of Iomedae (largely analogous to Pelor, if you squint and tilt your head just right) and a paladin got to help decide whether to free an entrapped devil to make the dungeon easier; the consequences of that decision (and how they attempted to mitigate the possible disaster) are reverberating.
