V is somewhere in between really great shows and really crappy shows.
The show has high production values, lots of action, lots of plot twists, and decent acting from most of the main characters. The premise is interesting, and it is marathon-worthy as the suspense keeps you going and wanting more.
On the other hand the show is very flawed, there are many plot holes and unrealistic drama. Too much of the show just doesn't make logical sense and feels sophomoric.
If you go into it without having too high of expectations you'll probably find it quite enjoyable. If you go into it looking for the next great show you'll probably be disappointed.