Perfectly sized, thin, signature Patek complication, gorgeous and balanced - I think it may be one of the most unanimously loved modern Pateks.
The only knock, in terms of collectability, is that it was made for a long time. But a first series 3940 in great condition is a beautiful thing.
I know this is blasphemous, but I prefer it to the 5970. The PPC is of course THE Patek complication, but I think wearability is a paramount consideration and the 3940 disappears on the wrist.