The Eldar codex has always been a tough one to beat. Excellent shooty and some good assault based units. However, it has been impression (and that of many locally at the club) that if the Eldar player knows which codex he is up against next he should never lose because the Eldar codex is perfect for cherry picking exactly what you need. There are some units that are pointless, guardians and swooping hawks - I'm looking at you. Dire avengers are far far better option that guardians and warp spiders are a better choice than the hawks.
Under 6th I think that they are still very good but Eldar players are grumbling that they now have no anti-terminator weapons anymore and that banshees are now pointless. I disagree. Unless you are facing Dark Angels or Space Wolves you won't see that many terminators. Scorpions and banshees can still tear through regular troops with power armour or worse.
When this codex came out all you could hear about was how overpowered and cheesy they were. What people weren't realising was that this is very much an elite army meaning that they are expensive and you can't take everything. This is their balance. Anyone playing against them just needs to do their best to avoid close combat with them and gun them down with massed firepower, same as you would any power armour type opponent.
6th edition has made them better in close combat and given them some more useful options. For instance, until 6th I never saw anyone take the GK Librarian. Now they get a lot more play. The new edition has forced GK players to adapt a lot more than any other army has needed to I think.
Under 5th this army was massed firepower and tanks. Under 6th it has become more mass firepower, less tanks and a lot more heavy weapons. The IG codex irks me at times because of the sheer number of shots that it can put out which can whittle down even a full power armoured army. It feels overbalanced and that seems to have been made worse by 6th somehow.
The biggest shift has definately been that vehicle heavy lists have been pushed to the back as they'll take so many shots that they'll fall apart under the hull point rules. The army is very much now a footslogging or stand and shoot infantry army with some limited heavy tank support. Overall the codex got worse in some areas, better in others and so basically stayed the same.
My thoughts on the Necron codex has been that it is a nice codex but isn't anywhere near as powerful as people want to make out. It does now have some good options but really it felt fair compared to everyone else. The only truely stupid additions was entropic strike, an ability that should have been rethought into a different mechanic.
Under 6th edition Necrons remaind balanced and are quite easy to defeat. This only changes when they take allies to cover their shortfalls. Much like dark eldar, this is what they need to do to remain competative IMO.
I'm honestly not sure what to say about the Ork codex as I rarely played against it under 5th and I have had no chance to do so under 6th. The only thing that really comes to mind about the army under 6th is overwatch. When overwatching their ballistic skill doesn't change all that much so it's just like any shooting. Plus, burnaboyz when using their weapons as heavy flamers auto hit d3 hits per weapon. I can fully see certain weapon options and bikers being made more use of under 6th edition.