Welcome to Librarium Online!
Personally I think that they are good units to have since you don't have to move them and they can just shoot throughout the game. The only thing is that Eldar have such good heavy choices (W.Lord, Reapers) that most people would rather take those instead.
Still I think it might have a place in certain armies. Iyanden and Biel-Tan comes to mind. Iyanden don't have to worry about replacing Wraithlords and the 48'' range lets you focus on Sniper Rifle squads or threats to W.Guard/W.Lord. As far as Biel-Tan is concerned the army is very specialised so a weapon that is S6 Ord will fit well since it pins and handles transports and light infantry.
Pairing it up with say Fire Dragons would be very nasty indeed since you could potentially pin units for the Fire Dragons to get an extra round of shooting off.
dun let the strength 6 fool you, its a nice weapon, but takes a heavy support, and is a guess weapon. My suggestion, stay with the wraithlord, and if playin iyanden, then take falcons and/or fire prisms
unless you are a very good guess