1) Wave Serpents are either gunboats or transports, but not both. If you use them as a transport, you need to Jink to keep them alive, which basically removes them as a shooting threat. If they're a gunboat, then they can't afford to Jink to add to their survivability. Therefore, a gunboat should take a shuriken cannon while a transport should take holofields and perhaps a ghostwalk matrix. Unfortunately, I built all of my Wave Serpents with shuriken cannons, and I run mine primarily as transports. As it goes, I guess a 10 point tax on the Wave Serpents I use as a transports isn't bad. The extra firepower does come in handy if I get to fire an opening salvo before delivering the troops or if the Wave Serpents survive after dropping them off.
2) Warp Spiders are almost obscenely fast, and their firepower is impressive. Their price isn't even bad. A squad of 10 costs less than I spend on a Tactical squad with Rhino. However, all that speed tends to attract a lot of attention, and while they're durable by Eldar standards, they cannot stand up to much firepower. Placement is key. You have to plan multiple places to hide every time you move them up to fire, depending on how far their final jump is. Also, don't be tempted to move the full distance that you roll every time. Move as far as you need to in order to reach your target and get away to safety.
3) Swooping Hawks seem very situational. Against an enemy with light infantry, they are worth their weight in gold. They simply tear through T3, low armor infantry between their grenade pack and absurd number of shots. So far, I haven't been able to use them to deliver haywire grenades, but that will come in time. They do just bounce off heavy infantry, so if you're fighting them, just try to keep them out of the way and score objectives. I have learned to always deploy them and Skyleap them so that you don't have to worry about them staying in reserve.
4) Massed Dire Avengers have been working out well for me. However, they are very one dimensional. They chew up infantry, even heavy infantry, and they can put wounds on monstrous creatures with Bladestorm. Don't rely on it too much though. 10 Dire Avengers will average 2 automatic wounds when they fire (Giving them Guide and Dooming their target ups that to 4).
Fuegan has the advantage of pretty impressive, though short range, shooting. He should be a monster in close-combat, but I haven't gotten him there yet. He should be durable enough to get into assault fairly reliably though. He'll definitely require some more play.
Asurmen has done the best so far. He doesn't have the shooting of Fuegan, but he does pretty well in assault with S5 AP2 attacks. His real benefit is multiple Warlord traits. With good rolls, he's almost as good at supporting your army as a Farseer, and he can be extremely durable. However, you pay much more for him than a Farseer, so you need to get him into assault at some point if he's to be worth his point. I find him very useful as a counter-attack unit. Otherwise, Dire Avengers are a prime assault for enemy assaults.
6) Guardian Defenders are surprisingly good (to me at least). They provide a nice unit for holding objectives, and their heavy weapon platforms give you some decent ranged firepower. Their real value is when the enemy decides to come close to what appears to be a vulnerable Troops squad. The Guardians have the same firepower as Dire Avengers at close-range, and there tend to be more Guardians. A full squad will make a mess of most targets.
7) I know this doesn't come as a surprise to anyone, but Farseers are really useful. I started off playing without psykers, mostly as a holdover from my Space Marines. As much fun as the Phoenix Lords are to use, they don't provide the support that Farseers do and they cost quite a few more points. Guide, Fortune, and Doom can give you more damage output or resilience wherever you need it. I've even had some success with Mind War and Eldritch Storm, though Executioner has always left me a bit cold.