I caved in and picked this up over the weekend and managed get a couple of hours in. First impressions I think could be summed up as positive but not without faults. The game looks and handles really well - handling wise this definitely falls in to the sim side of things but controller support is pretty decent if occasionally a little slow when trying to catch snap oversteer. I'd love to give this a go with a proper wheel set up but at over £200 I don't think I'll be buying one just to give it a go (though I do miss being able to race PC sims). I can't speak for the accuracy of the real life tracks but they look good and some of the fantasy tracks are brilliant to drive.
My biggest gripes about this though are, relatively speaking, quite minor so I hope they get patched at some point. The paint shop / decals possible being the biggest - in all the racing games I've played I have enjoyed being able to customise my cars livery. Obviously you can do this in GTS but there seems to be things missing / broken about it. There isn't (or at least I can't find it) an option for mirroring sides of the car so getting the left and right side symmetrical is going to be interesting. There are also bits of the cars you can paint but can't apply any decals - this isn't small bits either, where I've noticed it is when trying to put decals on the front bumpers. I could somewhat understand if it was small intricate area like wing mirrors but being unable to custom paint from bumpers will ruin a lot of designs. On the livery front, I really liked Forza's market place where you could sell or buy liveries but GTS's appear to only be the ones you create.
Unlocking things by miles is an oddity for me too - it adds a second currency to the game and I'm not sure why. It means you have to play to be able to unlock stuff but you have play to earn credits too so Maybe a change to be able to unlock things with credits or miles?
Finally, while it was well publicised before hand about there being no career mode I do miss it. The driving test / licences are enjoyable enough, I do like having a progressive career mode.