I noticed a few people (mainly PAL users) asking about the F599 lately. I can understand how everybody feels, being hungry for more content. So this will come as little relief but I can assure you, it's awful.
For the record, I have the NTSC version of the game and unlocked the F599 with the Gamestop code long ago. Back when I first unlocked it, I did a few installation laps at several different circuits but never went for any serious lap times. Since then, I haven't spent a lot of time with it. You're really not missing much. I think Eutechnyx did a poor job on the modern front engined V12 cars. Although I never drove one in real life, I'm pretty confident to say the F599 in GT5P is a much more realistic representation of what it would really be like to drive the car. In execution, it's actually very similar to the 550M/575M. It's big, heavy, understeers heavily and is tricky under braking. It does have very impressive power though. And it sounds nice.
But I can assure you, if the real car drove like this, nobody would buy it.
I did about 6-7 laps of Mugello with it last night and really made an effort to set a quick time. I never really put in a 'perfect' lap. Actually, I'm not sure if I would even call it a 'good' lap since it understeers so much, you have to force yourself to slide it around to get through corners quickly, but my best run was a 2:12 flat.
I'm sure with more practice I could drop a few seconds. But the driving style required means to do it consistently would be tough. (At least for me). None of the times with the F599 show up on the leaderboards so you'll have to take my word for it.
Either way, it's a good 12-15 seconds/lap slower around Mugello than the F430 Challenge. On the main straight, braking at 200 ~ 210 meters (necessary for this car) you can hit around 265 kmh, so it's quite fast in a straight line but suffers in braking performance and cornering speed.
Running a race with it would likely be a "messy" event with a lot of contact, much like with the 550/575. But there IS an option for NTSC users to open a lobby with it. I may try it sometime this week if I have time just for kicks.
Of course, few people seem to open or venture into lobbies in anything other than the F430 so I may be sitting there twittling my thumbs. 
