Hemlocks are good as a fire soak and the D-Scythes means it can't be ignored like some other things, but it can't be the only threat on the board. Like all Eldar units, it is how it fits with the rest of your army, they don't do well, if it's the only big threat on the board.
Conceal is great when it goes off, but it is very hit and miss, I've played a few games where it just doesn't go off at all. I will also add that when conceal goes off, people redirect the fire at something else as they don't really want to waste the shots. Additionally there is a fair number of 8" autohit weapons, they will munch it, so be very mindful of where you place it on the board.
I field 1 at 1500 points, 2 at that point level might leave your army unbalanced. I wouldn't have a problem fielding 2 at 2k.
I'm currently toying with the idea of using Horrify, Mind Shock Pod, and Shadow Spectres, maybe sneaking in a Death Jester as well lol.