Bought a Z1 helmet 8 jumps ago, and I'm loving it so far!
It's warm, the visor doesn't fog up, and there's a lot less noise from the wind. Because of this I feel more focused during the jump, hear my audible altimeter a lot better (there are pockets for 1 on each side of the helmet), and cold air doesn't bother me nearly as much.
The helmet itself is basically one-size-fits-all; the actual size of the helmet is determined by the liner (fabric) on the inside, which can be replaced. I've heard some people that had some trouble with the sizing - apparently not every shape of head fits perfectly in this helmet. Would I recommend it? Yes! However, if you're buying it online I would recommend trying one on before you order it (either from a friend, or in a store).
The only downside, is that it's a bit tricky opening the visor with gloves on (although not impossible, just takes me a few seconds).