Wow, hi all, this is my first post here so I figured why not jump into the fire and toss some thoughts in.
Just to give you all a little background I have been racing cars of some sort or another for about 13 years including open wheel cars and recently I've gotten into Rally. I own a VW Fox race car I run in endurance races, and as well I do a lot of karting. I've also been playing GT since only GT4, I have a bobearlracing.com seat with the old style Logitech GT wheel (I'm kinda broke right now cant afford a new one).
GT5-wise I generally like to use the right car for the job, I don't really bother modding my cars but instead buy something that fits each race type. I also run with no driver aids most of the time (except in that stupid top gear race with the lotuses that was designed by satan himself).
So am I disappointed? Basically, no.
I do have a few criticisms, such as the rally handling which seems to be just a bit off causing me to have to adjust my rally driving style to fit the game. I did the first rally challenge with a stock Ford Focus ST to closely mimic the Fiesta rally car I have driven, and while they should have handled the course similarly, the game makes the dirt and snow feel way less stable than it is. Its nothing I couldn't adjust for, just took some practice and a lot more concentration. I was a bit disappointed in that though.
I'm also disappointed that the AI is still clueless many times. It does seem to mimic what you see form your b-spec driver which is kinda cool, but it just doesn't operate on a higher level that you'd hope for.
Rubberbanding is DEFINITELY happening, for example I did the Euro hot hatch race and the Festival Italia, running an 06 Alfa 147 and while I would rip away from everyone on the turns, once I hit the straightaways other Alfa 147's would catch and pass me. I don't even bother trying to block anymore because about 50% of the time they spin me out if I do that. I know from real life theres no way an identical car is going to catch and pass me on the straight if I've entered and exited the previous corner faster and I'm that far ahead. At least they don't run away from you and you can eventually put them far enough behind you. But it can get frustrating.
I agree on the standard car models, while some don't look so bad (my Mercedes 190 E Evo II looks pretty good), I drove a 91 Nissan GTR yesterday that looked like It was straight off the original GT on PS1!
I've never driven Forza so I have no basis for comparison there, however I can compare to NFS: Shift and I can say with certainty GT5 is in another league. Yeah Shift may be more polished and structured, and of course you can mod any car however you want. But as maddening as the GT5 AI is just holding its line, the Shift AI is out to kill you. The first turn in that game is always a big smashup, and the AI drivers will intentionally smash you out of the way to the point its maddening. I got nearly to the end of that game and just gave up, I couldn't take the frustration anymore. The handling in it while good is no match for GT5 and many others, it feels like the cars are purposely tail happy, and the fact they even reward you for bashing your opponent out is stupid as well. Finally the "original" courses in GT5 are well designed, the ones from Shift are all pretty horrible. Some of Shifts courses are even too tight for the bigger cars to race on. I hope these things are improved with Shift 2, hopefully the FIA GT license will be put to good use there.
One other thing, GT4 just seemed so huge and unfathomable, with the dealership races and stuff like that. GT5 just doesn't, I feel like I can almost see where the end is, where GT4 the end was never really in sight. I see why they did some things, but I find myself wondering what was going on for so long, because I don't think they were adding events (if you recall Prologue even told you what regular GT5 event each race you were playing was from). It just feels overengineered, like they were trying too hard to anticipate everyones desires. They should have released it 2 years ago, we probably would have been more impressed, instead the hype has killed it for us all.