Your blending is good, although it's incredibly difficult to do this style of photo manipulation and have it look completely "right".
The only technically issue I have with this piece is that having an inconsistent light source is one of the big give-aways. The first thing I noticed is that the light on the Green Lanterns is coming from the right side of the picture (off of his left shoulder) whereas the lighting on the Doctor is coming from the left side of the picture. This is one of the reasons it can take so many hours of hunting through stock photos to find JUST the right images to use.... the angle of the shadows needs to (ideally) match up.
Otherwise, fantastic job, though... plus, I *LOVE* the concept.