Although I havn't played him yet, I have had the chance to play a bunch of games with him on my team. He is like a jacked up version of Dark Lady (from HoN) and Spectre (from DoTA). Jumping at every gank with complete darkness to the opposite team, with quite a bit of damage. He does look quite badass as well. Once I get him we'll see how fun he is... 3k IP later.
I'll do a good gameplay video with my awesome free recording technique, check it out if you havnt yet