in the end a reveiw is just someones opinion not fact at all there are tons of games people miss or dont give a chance because the reveiws were not good its a matter of trial and error and knowing what you like or what you look for in a game
To extend that, there are games made by companies that don't get the same exposure or creedence that more popular games may get, for better or worse, and everyone can pick one out.
I don't do pre-orders myself, I don't like that they now come with extras that may be already finished and, like GRID 2, were announced and shown at the same time as the regular game. The only games I preordered were GT5 and GT (PSP), so I may not be the best example.
As for determining how the game is, reviews don't do enough. If you want an honest opinion, look for critics. It may sound backward, but anyone can see the good/bad on the surface, a critic usually dives straight to the core of the matter, pointing out good and bad, usually more good than bad but that is usually what we find ourselves after playing for a while.
Still, the best way to determine your state of enjoyment is, simply, play the full thing. Demos are nice, but like Hollywood "Blockbusters" of the last few years, can feasibly consist of the only good bits in the game. My advice for the single player focused; rent if you can, buy used if you aren't sold on it. If you do buy used, hunt for the first month because after that the impulse-buyers will have taken it back for something.....I'll stop there.