Quote from Tharp
Depending on how common they are, they may not be encountered enough to where the creepiness factor wears off. I think that they will be a mob that you won't fight often, but one that will occupy part of your thoughts if you spot one. You may be carefully watching it while going about your business, but very rarely will combat actually occur. They will be more like an monitored potential threat, rather than a constant annoyance.
I think it may depend on whether they spawn in caves. They don't seem like a cave mob to me. Then again, creepers don't really fit in in caves either. They'll definitely be more of a danger in caves when they'll be difficult to spot and you're more likely to put your cursor over one on accident. Out in the open, probably not much of a problem.
One question on my mind is what happens to them during the day. Do they burn or just disappear? I think it would be creepier if they used their powers to teleport away just as the sun comes up, or if they headed for the nearest hill or row of trees and disappeared as soon as they left the player's sight.