I don't think anything will match the levels of excitement I had for GT5, following details on the game until it finally released. Then I finally got a hold of it; I swear that I had well over six months of playtime logged into GT5 throughout three years since I've played it. Not once was I disappointed at all with the game.
The presentation was just something to behold. I could sit at the menus for hours just staring at some of those little segments featuring your car or driver avatar. The one featuring the guts of an engine always comes to mind. To me, it just truly felt like the next generation of racing was here, at the time of course. I didn't mind the standard cars one bit; I wanted to collect them all, and GT5 made it interesting as to how you collected them. It just kept me playing forever and ever...
As I leveled up and progressed, there was something new and exciting to behold. Let's just say that reaching Level 30 drove a hype bullet straight down my throat; just when I thought I saw everything about this game, I see the X2010 Challenge and go through a daze. I don't know how many laps I did to get a Silver award overall for the challenge, because I wanted my own X2010. After much practice and frustration, though, I managed to get it, and the game then became a cakewalk at my own choice...
I don't recall just how many hotlaps I did with the X2010 at Cape Ring; I mean, that course is just so perfect for it. The flow was so smooth with lots of deep bends and twists that required snap-fast reaction from the driver, but you still maintained such a high speed. That was often a pasttime for me after school or work was just to sit down, take out my X2010, and go hotlap the other Ring.

The other obvious thing would be collecting cars of course; I managed to amass about 1187 by the time I was done playing.
I'm going to miss GT5 and all of the well-spent time I had playing it. I want to go back to it, but being used to GT6 I'm curious to just how much the two games differ. It was a great ride, and still is for some, but Sony and PD are making us stop the ride for good unfortunately. Hopefully GT6 can redeem itself in the future...