Good Necrons could mean many things: good Necron units, good Necron army lists or good Necron players. For all of these the answer, according to my experience, is yes. Each unit is good in its own right. They have the single most powerful regular model: the Nightbringer and the most resilient regular vehicle, the Monolith. All of the models have the capability to have a good win record. It still comes down to dice rolls and tactics.
As it stands, no, the rules are severely outdated, though there are rumors of necrons being updated sometime before the end of 2011. If that is the case, then the Necrons will be a viable army to play again.
All are good at diffrent things Tau: Great for long range but not much else Tyranids: Great for swarming in close and personal and ripping them up Chaos: Good all round troops well balanced for beginners Necrons: Think you kill them they get back up! damn things don't die! They have great leadership as are robots Overall id say necrons or tyranids. i personally prefer necrons but that is just me.
Theoretically, any faction can team up with another, as long as you have a good enough reason. Technically, Necrons and Chaos are enemies, so they would never 'team up', per se, but one could create a feasible story in which they both work to exterminate the Imperium. They wouldn't be working 'together' (because Necrons would kill any living thing--including Chaos Marines--on sight) but they would have the same goal.
Answer Probably Necrons.
Yes they do.
They Are The Second Best 40k Army Below Tau And Just Above Necrons
Start with half your necrons in reserve deep strike monilths into enemy lines and let the the necrons pour out
Get a night bringer and pariahs.
Necrons
If the blood angels have a couple of squads of Death Company with jump packs, Astorath, and two furioso dreadnoughts with drop pods, you can grind the necrons in assault. But if the Blood angels don't make it, the necrons will flay them in shooting. If a C'tan is there, forget assault, Sons of Sanguinis.
Technically you can play any army against any army, and any army with any army, except where rules specifically state that you may not (E.g. Black Templars may never ally with an army containing pyschers) It would however be extremely unusual, but so long as you have a good background story, it should be fine.
As a Grey Knights player, shoot them as much as possible before they get to you. With the new rules it is very hard to beat them. Blow up any transports and make sure to take out paladins, special characters and terminators.