It's brilliant. Go see it! I'll try to write a longer review for later and just write a few quick impressions for now.
The theatre was almost full, which is encouraging given that the film hasn't had much publicity. I really enjoyed the movie, and my impression is that the rest of the audience did too. This film has some very funny scenes, and the audience laughed out loud several times.
It has a great cast. Keanu, Woody, Downey Jr, and Winona are a great group of actors and play off of each other very well.
I think that the rotoscoping works very well with this material and is necessary to make the film work as well as it does. It wouldn't be nearly as good if it was just shot conventionally.
It has a lot more dialog than a typical big studio release, and the pacing feels slower. That's OK, because the dialog is great. The soundtrack is very good and works very well with the film. It gives the whole thing a 1970s feel.
It's a unique film, and different from anything else out there. I think that is why the critical reaction has been mixed. I think that the critics don't quite know how to review it and weren't quite sure how it would play with audiences.
I haven't read the book so I don't know how a PK Dick fan would feel about the film.
Will it do well at the box office? I think it could be a cult hit, although some will find that it is too much dialog-driven. I would be surprised if it flopped, and it would be a great shame if it did. The main problem I can see is that it has to fight for screens with all the other summer films out there.