What do I mean? Well, let me explain.
Again, let's use the standard candle of vanilla terminators. Now you might ask, why? And I would understand, because as a rule of thumb, you should never try to compare units cross army since it will only lead to frustration in the end, but primarily because armies are only balanced against themselves, not each other. Terminators, though, have the benefit of durability, very strong attacks, cost effectiveness, and even mobility (land raiders). They can do that complete package for less than 500 points.
That being said, what are we left with? As discussed previously, bike councils offer good durability, but at a premium cost and with no significant killing power. Shining spears offer good killing power, but with reduced durability (compared to the council) and still at a ridiculous cost. There's also the factor of their max squad size. That leaves us with Howling Banshees and Striking Scorpions. Both are good units and each has a specialty: Scorpions are meant for horde or swarm style armies, and Banshees are meant for pretty much everything else. So, when tournaments require you to have a submitted list that you can't change out throughout the tournament, how are you supposed to prepare?
Based on the math, against all opponents, your best choice is the Scorpions. They offer some durability with a 3+ armor save (fortune is the win here), but still suffer from only being toughness 3. They have an increased number of attacks with the mandiblasters, and the option for a power fist. These guys will do well against horde/swarm armies, as mentioned earlier, as well as tactical marines and other similar units. In the grand scheme of the game though, Scorpions may seem lackluster, but there is a lot going for them. Their biggest asset is their initiative 5. Against most armies, this means you're going first with roughly 40 attacks at WS4, and with the +1 strength from their swords, they'll be converting wounds nicely. With farseer support, these guys can really hurt. They do have their issues though. First, they're not fleet. A trade-off for better armor essentially, but they can purchase move through cover cheaply. There's still the issue of toughness 3, but a re-rollable 3+ armor save (fortune) balances that out, but their biggest weakness is the metagame. There are just so many marines out there that fielding these guys is just a little impractical. While I would always bet on the scorpions beating out a squad of tacticals, they aren't capable of killing it fast enough to prevent a counter from a more "elite" unit, like terminators. With the 40K gaming scene being saturated with marines players, Scorpions just don't get a large enough opportunity to shine. Another good example would be the new inquisitorial henchman from the new GX codex. The death cult assassins are scary. Hopefully, we may see the Scorpions moved to a troops slot position with a new Eldar codex in the future (they did it with incubi in the DE dex :) ). That may pave the way for a more elite assault choice, or possibly give them a chance to buff up the Banshees.
Speaking of Banshees, they are our next topic. While I said that Scorpions are the overall best choice, Banshees are the most practical. They hurt marines players where it matters: in the armor save. What's critical though for these guys to function is access to a transport, which presents it's own issues, but I digress. With only a 4+ armor save and toughness 3, bolter fire is just terrifying. If you do have to move these ladies on foot, make sure to abuse the cover save rules and "fleet" as much as possible. Once you do get an assault off, just watch your opponent melt. Properly run, again with farseer support, you can statistically kill a whole squad of tacticals on the charge, as well as a squad of regular termies (based on an average squad size of 5, and no storm shield). Despite their being frail, they do have an good defense of having a srong offense. You should kill enough models to wipe out an entire 5 - 10 man squad, or leave only a couple models behind such that the return attacks won't really matter. With the guarantee to always go first, this is an effective defense method. And as far as the metagame goes, pesky rules like Feel No Pain, which is fairly common now, and will be even more so when Necrons hit, won't matter. So with Banshees, they are the winner for now with current game trends, but make sure to avoid swarms at all costs. Killing 10 out of 30 orks isn't that great an accomplishment :P
So what is the current status of the Eldar as far as assaults go? It depends. While lauded as their greatest asset, the specialization of the assault units makes them unwieldy to play consistently. And while I know that this isn't unique to the Eldar (Ork boyz can actually walk all over terminators just through sheer attrition for example), it's just painfully more obvious. Also, while I will stand by Wave Serpents till the end of days as the best transports in the game, the lack of assault vehicles as well as just the general rules exacerbate the situation. If spinning a vehicle didn't count as movement for the purpose of assaults, that would make things much easier to work with as far as Eldar assaulting. If you do plan on running either of the assault units mentioned, knowing and understanding their strengths and weaknesses is critical. In all situations, no Eldar unit should be operating on its own. For example, there should always be farseer support, or the scorpions are assaulting after a bladestorm to mop up the survivors, etc.
To sum everything up, assaulting for the Eldar is tricky. Each type of assault unit has little idiosyncrasies that make it good vs some, and bad vs others. You should obviously choose what you're most comfortable with, but if you plan on competing, Banshees seem to be the best choice right now with so many marines players. I'd love to hear some other thoughts out there.
P.S. I just realized I forgot a whole other unit, but I can sum them up very quickly: friends don't let friends play harlequins.