A bit harsh, I don't think anything PD have ever made comes anywhere close to deserving being called garbage, but that's just my opinion....
I agree it would be a waste of cash to buy the game now (only 20 more days!!

), but after getting '100%' (by which I assume you mean all golds?), I continue to get alot of game play out of it, not least because I have a DFP (Driving Force Pro) and Prologue supports 900 degree mode...
The object of the license tests in Prologue are slightly different to GT3... alot of tests you only have to complete to get the Bronze, whereas to Gold them all is somewhat more difficult.. but even after you gold a test, there is much room for improvement... a gold will score you 100 pts (a taste of the new GT4 points system), but you can improve every test up to ~300 pts... so there is no '100%' in the game really...