First off, I'd like to say I'll be covering the Wraithguard and Wraithblades in troops as I think they are nearly always taken with a spiritseer in order to convert them to troops.
I may be alone in this belief, but with the WraithKnight in Heavy Support I'd like to see the Wraithlord moved to Elites. It would reduce the burden of Heavy Support, keep it more slot appropriate (compared to a dreadnought) and could see Wraithguard simply moved to troops.
None of that really influences my thoughts on these units, but I think over this whole review it's important to disclose my thought process in regards to the dex overall as I go.
This is a unit that, at first, felt like a missed opportunity, but after actually using them I find them quite good. They're incredibly fast thanks to the acrobatic rule.
Game Play - Other players WILL underestimate Banshees in a serpent... why? Because they'll forget about Bladestorm. Any time you can deny the enemy an armor save you'll start putting real pressure on them. Bladestorm has made the Eldar pretty much the masters of short range firefights, second only to the Tau. Deploying them out of a Wave Serpent gets them across the board insanely quickly. However, using them requires precision and thought. They are not ideal for sending in to dug in opponents and clearing objectives. However, they will tear through a unit trying to cross the board or units that are threatening to assault your units in your deployment zone. Banshees tend to benefit more from other units providing synergistic help. For instance Jinx on terminators makes them fear the charge of banshees (not to mention that they should be wary of any unit with a shuriken weapon). While it may be tempting to put a Spiritseer that can boost their strength in the unit, or an Autarch to give them a combat cornerstone to build on, but doing so will lose you Acrobatic, so I would advise against it.
Phoenix Lord - Jain Zar is nasty with AP2 attacks and shred, but the Triskele makes her buff the units shooting considerably, not to mention her warlord trait giving them an extra inch on the run, and believe me, every inch counts and let's not overlook her ability to tank wounds with her 2+ save. Her mask gives her the best chance at dominating on the charge, and she's generally not afraid to issue a challenge, though be aware the true monsters of combat, like Lysander, or any Chapter Master with a storm shield and thunder hammer will probably win out in short order. Use your mobility to pick your fights carefully.
In 7th: Banshees in a Wave serpent have lost 2" from their disembark, but they're now a scoring unit, which makes them flexible, but capable of quickly sprinting to an objective. In an edition that is more geared towards shooting the banshees can hold up pretty well in part because they can do some damage with shooting and are likely enough of a threat in combat that the unit you want to assault won't assault you first.
A lot of people freaked about the leap in points for Fire Dragons, but they needed a raise in points, and I think the return of their 3+ save makes them really worth it, and their firepower is still impressive and reliable
Game Play - I've seen people use them on foot alone, but I find they're such a threat they need some type of transport to get to the biggest threat priority. When tanks explode they have a real chance to survive the blast now... assuming they don't take a D to the face from blowing up a super heavy... but, really, in most cases it's worth the sacrifice.
Phoenix Lord - I have not used Fuegan in the new book, but I have one friend who refers to him as "boss" and absolutely loves him.
In 7th: AP 1 is even more valuable and the dragons are reliable. They can do quite well on their own, as aspect warriors should, but Guide and Doom can help them all but guarantee to liquify whatever target you throw them against.
This is the unit that brought me into the game and in the new codex they received had a lot of the old upgrades that were all but required to make them usable built in from the start. The mandiblasters operate a bit closer to older versions, and while the loss of the bonus attack might make me frown it's well worth the trade off for them being so all around good. Also... powerfist that strikes at initiative!
Game Play - Scorpions are incredibly flexible, but for me I tend not to put them in vehicles as it drives the cost of the unit so high. Using them to infiltrate, usually in ruins, can force your opponent to either cede ground to them or move to deal with them. Either through board control or directing your opponent's hand it is a potent option. The ability to outflank should also not be overlooked. Much like the banshees their firepower can really surprise people, and don't forget they have plasma grenades, so they can get a Str 4 AP 4 blast in that volley as well.
Phoenix Lord - Karandras is my favorite. He has a power fist that slams down at Str 8 at initiative and gets a bonus attack for a second weapon. Well worth the loss of the ability to go to ground.
In 7th: They're still awesome and they can now score, which should not be overlooked on a unit with stealth and a 3+ save. If you don't have Karandras they can infiltrate to a mid board objective in ruins and just go to ground for a 2+ cover save on a unit that most people won't want to charge to try and clear out.
This unit is a missed opportunity, not in that they are a bad unit, but in that they don't feel particularly up to snuff compared to the standard units and I feel that keeping them in both the Eldar and Dark Eldar dexes has prevented them from being appropriately moved into the new edition as GW strives to keep the unit the same in both books, thus making them stagnate, however the bladestorm rule was a boost that eased some of the restriction from the unit rules being unchanged. The addition of Battle Focus makes Harlequins feel a little odd because they feel like a unit that doesn't have the same personality of either of its parent dexes.
Game Play - This unit is incredibly fragile, but they're not slowed by difficult terrain, and can be the unit that takes on those 2+ save units or 3+ save units in cover that your banshees shouldn't be charging. Always, Always take a Shadowseer.
In 7th: Like the other Elite selections the Harlequins are scoring now. Ultimately the change to the psychic phase has quite the impact on them. When the unit is in combat he provides spare warp charge to the rest of your force, and if you cast on a single die there's no chance your Shadowseer will perils, and even on two dice you are half as likely to perils now that double 1's don't force the perils, and when it does happen it doesn't automatically kill your model.
Eldar elites are much more tempting in 7th now that they can score. it really hindered them in the past.
Banshees: 10 in a Serpent (vectored engines) with Jain Zar and an Exarch. Throw them head long at a vulnerable part of your enemy's lines.
Fire Dragons: 6 in a Serpent with an Exarch with Fusion Pike and Fast shot in a Falcon (vectored engines). Throw them head long at a unit that absolutely, positively has to die.
Scorpions: 10 with Karandras and an Exarch. Infiltrate and control a portion of the board, or outflank if you can attack a vulnerable enemy up close and personal.
Harlequins: 10 with a Shadow seer and a handful of Harlequin Kisses. Keep them in cover every moment you can and get them in combat with a priority target as quickly as you can.