I definitely thought the idea of an un-turned (of being vampire) sorcerer to be interesting. This gave Sebastian both an edge (who knows what the 'power' of vampire virus in his body could do to him!) as well as a curse because of what he must do to keep the virus at bay (it involved 'separating' his power, which made Sebastian not as strong a sorcerer he previously was)
I was also curious on why the vamps wanted to turn Sebastian. There must be more to it than just a revenge plan because Sebastian hunted the vampires under order of the Ministry.
As for Sebastian himself, well, one thing for sure, he loved his Mom :). It is needed to be highlighted for me because often, heroes in this genre have family issues! So it was refreshing to have one hero that truly will do anything for his mother. On another hand, Sebastian also needed to learn about everyone else's problem with him. He was not exactly an asshole, but Sebastian never did try to understand why other hunters hated his guts.
All in all, this was a nice start of an urban fantasy series...