a friend who's just getting started on Tyranids couldn't stop raving about how Harpies look ruleswise, so the two of us split the cost and conspired over the last couple of months to raise a force of half a dozen of them. I did the modeling, and made some mold for scratchbuilds, and we both ordered such things as wings, talons, bases, and guns from GW. (I lie, I had all but the wings and bases in my bits bin, though I did do custom bases)
After about a month and a half of prep, and two weeks of trying to pick a 2v2 fight, we unleashed 6 Harpies and 4 Flying Hive Tyrants into a 2500 point game. Our opponets were amused, and proceeded to die in a most sporting fashion, taking several of the Harpies with them. While the Tyrants did their best work on the ground, it's really the Harpies I wanted to talk about.
Pointswise, Harpies are a pretty even buy. They are neither ridiculously expensive, nor cheap. the threat they pose to the battlefield is significant enough that they aren't meaningless, yet even an army that wasn't expecting them can bring the things down.
completely terrifies opponents.
lots of blasts, all over the place.
highly mobile and able to be in the right location.
Not particularly tough.
limited to 2 shooting attacks per turn, with 4 attacks to choose from.
Able to skyfire, but carries only 1, mid-strength, non-blast weapon.
No GW model.
I like them, and will get some pics up as soon as I remember to take them.