Late to the party I know but watched this at the weekend and loved it. The vast majority of the film is with the new cast - albeit the underlying story is about what Egon was doing and the resolution of that. It was a bit less weird than it might've been going in and knowing it was a respectful homage to the original film (and to Harold Ramis).
Maybe a lot of me liking the film was the strong links to the previous two films and so when the surviving original three Ghostbusters turn up that was just the icing on the cake. Also, a nice touch that (like in the first film) it took four proton packs to defeat Zuul as opposed to just three (and off course gave Phoebe a nice moment to shine).
Of course I could just be getting sentimental in my old age!