I don't think I will stop playing GTS until GT7 comes around, whenever that will be. What keeps me coming back? Well, the game is brilliant. After working a long day, I look forward to driving some of my favourite cars around the Nurburgring, Interlagos, and Suzuka. And the new track in the recent update, Autopolis circuit, is quickly becoming one of my favourites. GTS is great. Driving feels great, especially when you opt for no assists (save for weak ABS), you need to be delicate with throttle and positioning, especially with the fast Gr.3 and Gr.2 cars. Unfortunately, I never got around to playing GT6, but the physics have improved a whole lot over GT5. Mastering a circuit in a nice car with the sunset on show is a great feeling.
For me, GTS shines in Sport mode. Polypony Digital took a huge gamble much to the dismay of their fans but I think it's starting to pay dividends. Good multiplayer is the future for sim racers, and now PD have a good base to really take Sport mode to the next level in GT7. For what it is right now, I think it is very good, but with some big issues. The penalty system provides the best sim multiplayer experience on consoles, but on the flipside, the penalty system is flawed and often punishes the victim. I cannot count the number of times where I've been punted off the track by a disrespectful Spanish or Italian driver, only to get a hit to my SR rating and a five second penalty. I really wish PD would do something about this. Some tracks are more notorious than others when it comes to bad penalty decisions, and dirty driving. But still, the number of memorable races that I have had outnumber the ones where I've been treated unfairly.
There are other issues with the game. I think the AI is still pretty bad, far too slow and unaware of your presence. But I like GTS as a complete package. The racing is fun (especially online), the car selection is good, and the game is very polished.