My quick review of Eldar units:
HQ:
Autarch - Can be useful in mechdar. Make sure you've got a solid squad to put him with (Warp Spiders, Seer Council, Jetbikes)
Farseer -
MUST HAVE. The only time you don't is when you upgrade to Eldrad.
Warlock Council - Expenisve unit. Wins plenty of games by itself, but face the wrong foe and you auto-lose.
Avatar - Good in Footdar. Avoid in mech. Must have if you're going
MC Eldar.
Yriel - Worthwhile, but not great. Use in a
CC unit.
Eldrad - Excellent choice. Take if you can afford him and you aren't running Jetlocks.
Phoenix Lords (all of them) - Look great on a shelf. Perform poorly on a battlefield. Avoid.
Elite:
Striking Scorpions - Works well in footdar, but not great. Avoid in mechdar.
Fire Dragons - Excellent unit! Must have in a mech force. Lack of delivery makes them hard to use in footdar.
Wraithguard - Meh. Usable in 10-man troop choices, but not great.
Howling Banshees - Good
CC unit in mechdar, but not great. Avoid in footdar.
Harlequin Troupe - Useful screening unit in footdar. Avoid in mechdar.
Dedicated Transport:
Wave Serpent - Excellent transport and gunboat. Possibly best transport in the game. Spam it in mechdar (obviously, it's your only choice

)
Troop:
Dire Avengers - Must have in mechdar. Usable but more limited impact in footdar.
Rangers - Not great, not even good. But, are a cheap way to park a squad on an objective. Be prepared for a single flamer to wipe out the squad.
Guardians (Defender) - Pretty bad unit. Only use if you have an Avatar to support. Avoid in mechdar at all costs.
Guardians (Storm) - Good unit. Gives
DA a run for their money in mechdar. Also somewhat usable in footdar.
Guardian Jetbike - Decent unit. A great way to get an extra scoring squad for cheap in mechdar. Also makes a good supplimentary unit in footdar.
FA:
Shining Spears - Looks good, but isn't worth the cost. Avoid.
Warp Spiders - Surprisingly effective unit. Works well in both mechdar and footdar. Not a must have, but a good place to spend extra points.
Swooping Hawks - Worst unit in the codex (apart from a few Phoenix Lords). Avoid at all costs.
Vyper Squadron - Meh. A cheap way to get extra heavy weapons if you're out of
HS slots.
HS:
Support Weapon Battery - It's an artillery unit. Enough said. NEXT
Dark Reapers - They can be good, if you're fighting foot-based Marines without drop pods. Avoid in a
TAC list. Avoid in mechdar.
Wraithlord - Avoid in mechdar. Usable in footdar. A must have in
MC Eldar.
War Walker Squadron - Excellent choice. Usable in both mechdar and footdar. Bordering on must have.
Falcon - Playable in mechdar, but typically is worse than a Fire Prism. Divided opinions among players.
Fire Prism - Possibly the best
HS choice. Not everyone runs them, but they're pretty close to a must have.
Night Spinner - (Rules are in a White Dwarf) Moderate to good. Excellent against horde armies, limited use against mech forces. Usable, but not a must have.