I have had some experience with them, in two games vs Necrons. First game was 1000p. I ran two heavy venom Harpies along with Warriors and a Prime and 2 single Biovores. I kind of screwed this game up, but the Harpies were remarkably hard to kill even when they were flying in the middle of the enemy's army. The second game was 1500p where I ran the same Harpy pair and a Flyrant with devourers. Those three flying MC's along with Biovores are awesome! He ran a quite Necron Warrior heavy list, so he had to spread out quite a lot to reduce the Biovore damage. Then those three MC's with their mobility, firepower and CC abilities of the Flyrant wreaked total havoc on one his flanks. When they were done, the Harpies flew straight into the middle of his army firing venom and Cluster spines at his Monolith, taking a hull point and a few nearby warriors and smash attacking it afterwards. Although I didn't manage to kill it, it showed the tank hunting abilities of the Harpies and he didn't manage to shoot them down afterwards. Next turn they flew onwards, both dropping their mines on the same nearby warrior unit, killing almost everyone (although 1/3 got back up again). Next, they shot away his warriors from his objective before he started to manage to hit them. Those S9 hits you take when you hit the ground hurt quite a bit... I won that game, and I have to say that it was a lot thanks to my flying wing.
All in all I think they are great, as long as they are used right. A single flying MC can be shot down, but when you have them in pairs or three, you might start to quickly saturate your opponents anti-air fire. And now with the hull points, the venom cannon can be really powerful, especially when combined with other things like their S10 smash attacks. I think two Harpies and a Flyrant plus Biovores to force spreading out troops is a keeper for my 1500p+ lists.