I've been focussing on my Craftworld and Harlequin armies lately, but I still feel I can chip in here.
The decrease in cover save really hurts the corsairs, as does the addition of hull points. Corsair Mech was a very viable list in 5th, and I feel it has lost quite a bit of its power now that the falcons will get glanced to death. Not being able to score from within transports is yet another blow to the mech list.
Corsairs also suffer from the same issues Craftworld Eldar suffer from in 6th, such as the boost to rapid fire and the aforementioned reduced cover saves.
Overall though, I still feel Corsairs are pretty much where 5th left them on the power scale. Now that the flyers are proper flyers (and Vector Dancers, which is incredibly awesome!) and the addition of allies really helps here. With corsairs you can basically combine 3 codices to maximum potential. That alone is very, very cool. What we need most now though, is some resilient units to soak up some of the incoming damage. Wraith constructs or allied Space Marines (preferably Deathwing or similar) really helps here.
The basic Corsair is pretty decent now with BS4 and a good selection of weapons. Just plonking down a couple of 5-man squads with Missile Launchers and Corsair Venoms to annoy the opponent costs next to no points at all, allowing you to take some more resilient troops to take forward objectives and get some of the awesome units available to us (flyers, Warp Hunters, Wasps, Hornets etc).