I love this game. I’ve been playing a lot of GT3, GT4 Prologue, and GT4 (including several early demo versions) these last two years and got me the Logitech Driving Force Pro wheel. Most fun I’ve had was when I joined up with lots of other GT3 fans for i.Link matches (sort of LAN parties, where you hook up 6 PS2s at a time and race each other for some really decent competition). I’ve organised some of them myself and went to a few in among others Germany, awesome fun. No matter which GT board you visit, you’re likely to come by my name at some point, especially if you try to find me. 
I imported the Chinese version of GT4 a while ago, and tomorrow or the day after I’ll get the European version. I’m not in a hurry, as my hunger has already been satisfied with the full Chinese version, but it will be nice to play this on the big widescreen 100hz TV (now I don’t because 60hz doesn’t look too good when converted to 100hz Digital Scan).
These days I’m no longer that competitive anymore not in the sense that I’m slow but in the sense that I don’t spend the time needed to get top times in the databases (e.g. http://db.granturismo.de/index.php - my current favorite). I still do pretty well in live racing, because the skills I developed in overtaking, holding a line, withstanding pressure and so on help, but I’m no longer a likely top 5 finisher I think - and with lots of old and new champs being drawn back in, I probably won’t be for a while either, but you never know.
I have a GT4 log up and running at http://forums.gttimes.com/viewtopic.php?t=9533, and this forum is really the first time I got into the forum thing, 2 years ago, and I just passed the 4000 post mark there.
I also am the author of a nice little bit of software that you can use to manage GT3 replay files with (upto and including GT4 Prologue, but GT4’s files are coded differently and I haven’t figured it out - I’m basically waiting for someone else to do that for me, I suck at encryption/decryption stuff - but once it’s straight binaries I can find my way around a file).
At the height of my GT days, somewhere this summer :D, I even wrote some columns on it for IGN (http://cars.ign.com). I got fed up with the game a bit after hosting the fourth installment of the Board’s Challenge, the biggest online GT event currently existing, with 11 teams of each 10 drivers taking part in 9 races, resulting in 170 replays. I guess I OD’d a little on the thing. 
Now I’m back to quietly enjoying playing this game occasionally, getting golds in the licences and filling up my garage, as the GT4 log shows. I’ll still definitely be visiting and occasionally hosting GT4 LAN Parties though, as that’s just too much fun and I’ve made a lot of great friends there. I’ll probably slip in the occasional online competition (where you race and then send in a file of your replay so people can check if you’ve been driving legal).
I did manage to play GT4 game online already through xlink kai as mentioned elsewhere, which was fun as I found a few drivers that I was a really good match with and we had some good racing. But it’s also a hassle and time-consuming affair unfortunately (partly due to problems with my HUB/Router config making it practically impossible to play with more than 1 other player), as well as a tad addictive, so I’m keeping away from that for the moment. We’ll see how it turns out with the PAL version.
Nowadays, I’m finding a lot more and healthier fun in playing DDR, and that’s consequently getting more floor-time (heh). But GT4 is something special, with it’s photo-mode and it’s 750+ cars and stunning realism. Sure, the AI of the other drivers suck, and there is no damage (though I see that also as a plus to be honest), but the feeling of driving the car is extremely realistic, the cars look awesome in the game, it has really improved my understanding of car-handling (in all its different guises), and those light-blue numbers* keep being addictive.
*those that play will know I mean the numbers indicating how much faster you were this lap than in your previous fastest lap, meaning that you’ve yet again found a small improvement in your line, braking point, etc.