If you've been reading the rumor mill lately you may share my excitement as to the upcoming release of the Tau and Eldar codices. Until these rumored books drop, or even more than rumors appear I'm trying to get the current books to work in new and interesting ways. Recently Godfrey and I had a game where we both took our oldest armies, allied in some 5th edition friends and went at it. While the game turned out to be a rather neat table for Godfrey I think I came out with some new knowledge as to how to run my eldar and how best to run the army's concept, at least until the new codex drops.
I would place the eldar codex squarely in the lowest quarter of codices for competitive gaming at the moment, with companionship from the orks and sisters. Sisters are arguably a very playable and decently strong codex, if forced into using units in very specific ways. Orks are very much the same as they were in fifth edition, with their waagh! changing but their codex adapting to a more infantry-centered metagame with numerous troops units. Eldar have seen more changes than Orks and seem to be in a similar situation to sisters, but with much higher prices.
no-ships would make some sense in their fear of Slaanesh, but it is monstrously destructive to a mechanized eldar army like mine. So I had to take my army back to its roots and run the seer council I loved.
I started eldar and eventually space marines to have an army that played differently conceptually than my tau's sit-back-and-shoot style. My eldar evolved along the same style as my tau, being mechanized infantry that tried to keep its distance, but did not start there. The seer council was one of my first buys for the eldar since they were both psychic and at first glance very potent in close combat, things that tau lacked. I soured on the unit after playing it a few times against dramatt's blood angels and Godfrey's templar when they first were being built and finding that a single assault marine unit was usually more than a match for a mid-sized seer council for about two-thirds the price.
I'm back to square one with 40k right now with the process of learning how my armies work again. Sixth edition's seems to have made the game slower and more time consuming while I'm waiting for codex updates to bring my armies in line with newer armies so that I can lose based on my own failing rather than the codex's. I know I need to improve my tactics, but I may have to get those honed on the new DA book so I'm not putting myself at two handicaps.