The Eldar codex is quickly losing steam. With the advent of the "power four" codexes (SM, CSM, IG, and Orks) the Eldar truly are a dying race. But they are a proud race, and those that play them have adapted.
This is the general strength of the Eldar. They have a TON of army entries and a vast array of weaponry to deal neatly with precise threats; from the four-shot scatter laser (up from d6 shots) to the armor piercing Bright Lance.
Eldar are a specialist race. What they do, they do well. So, it's important to have each unit doing what they do best.
This leads us to the Dire Avenger. In previous editions, he was an Elite fighter, but now finds himself as a Troop Choice. This changed everything. Before, the guardian was on the front line. But Guardians had a major issue, once within their spindly 12" range for shooting they could also be charged. Oh, how for years I longed for 18" shuriken catapults. That dream came true with the Dire Avenger.
One model can be upgraded to an Exarch, the Eldar version of a sergeant. And he can be given various equipment. Puzzlingly, all except one of these is close combat oriented. This opens up the danger of splitting function as explained above.
I have not tried them with any of their Exarch powers besides Bladestorm. They should fill one role, and they are definitely not close combat specialists. Their best use is as mid-range shredders. They have no special weapons, unlike marines who can tote along a lascannon for emergency situations. Suckers. They have not learned the lesson of staying focused.
For the same reason, I give my Exarch an extra Avenger gun. it doesn't look as cool as a sword, but definitely gives some extra punch in the shooting department.
A full unit of ten Dire Avengers, equipped as described will have 27 shots for the regular guys, and four shots for the Exarch, and that at BS5. That is enough to give almost any unit that is vulnerable at all to their shots a run for their money.
Two units bladestorming will evaporate even Terminators. Let's do the math-hammer on that: 62 shots will give about 41 hits, which will be about 20 wounds. Even on a 2+ save that's 3-4 Termies down. And a solid 6-7 regular Marines.
Hmmmm... Maybe I was wrong. Since the Avenger gun is an Assault weapon, the Dire Avengers can really have the possibility of mopping up a vastly weakened unit.
That same run of 62 shots against a T3 unit, like Termagants, will blend them into a fine puree.
I have a dream of running sixty Dire Avengers on one end of the board and seeing how much carnage they can inflict. Just once I'd like to blow somthing up with nearly 200 shots. Well, that's noob-think for you.
Now enter the Wave Serpent. This cuts down on the number of troops to be fielded since the Wave Serpent nearly doubles the cost of the unit of Dire Avengers. However, they are the beans to the Dire Avenger's rice: it can be mounted with an anti-armor twin-linked Bright Lance which can shoot at a different target. This is an especially effective combo in the now transport-rich environment of 5th edition.
Come to think of it, maybe TL Scatter Lasers might do better against transports just from sheer number of shots.
The Wave Serpents, when deployed in a pair or trio, can form an effective shield for your Dire Avengers. They can deploy, bladestorm, then the next turn get back on board for their turn spent reloading. And since they are right next to the entry ramp, the Wave Serpent can still move.