First of all - don't go by the japanese version for online play impressions. Lots of morons have uploaded videos on youtube of online play and it looks like it's jittery and sucking real bad. People are quick to blame it on the netcode or PD being morons otherwise.
This is not the case.
When someone from Europe or the 'states decides to play the Japanese version of GT5P online, where do you think the server is located? Where do you think the majority of the players are going to be located? You may have guessed it: Japan. Due to the limitations of the speed of light combined with inherent latency introduced by the insane amount of networking equipment required to get your packets to (and recieved from) this server in Japan, there's going to be a lot of problems. Sometimes it will be smooth, sometimes it will not be. Sometimes packet loss creeps in and that's when you see the "horror story" videos where cars disappear and reappear, or go 2000mph to a certain spot.
I can guarantee you when this hits stateside and people stop being retarded (and play on their country's version) it will be a lot better. Especially now that they're adding private rooms and things like that - looks like us GTPlanet freaks can get our game on without worrying about wreckers.
Online play will be great. Just don't be a tard and play on a version not meant for your country.