I think you should atleast play the game... if you are not able to return it after it's opened, then you can always sell it on ebay or exchange it, although both of these options will probably mean you'll make a small loss. If in the unlikely event you play it and don't like it, you can always try returning it to the store - if you make enough of a fuss and/or ask politely enough, you may find that you can exchange it for something else - that's what happened to me the only time I took back a game, despite being told that the store didn't accept returns. But, remember this... GT has always been a game that rewards effort - the more you put in, the more you get out. Although it is a very good 'out-of-the-box' game, arguably it is far more interesting once you have found your own particular niche - for some it is online, others specialise in photomode, drifting, endurance races or even simply car collecting. Put it another way, don't expect to get your money's worth in the first week, or even month.... but, if you keep it, and get into it, you will get your money's worth and more...
While my first impressions of GT5 are very good, I am (understandably) pretty biased - and I won't lie - there are already some features of the game I'm not overly impressed with, i.e. standard cars. But generally, this game has a whole lot to offer and I would challenge anyone who didn't think that GT5 is excellent value for money. Online promises to be brilliant, but for me the game is worth the cover charge for the premium cars alone... also, playing Gran Turismo in 1080p and the day/night variations at the Nurb and Le Sarthe are what I am looking forward to the most. Everything else - photomode, course maker etc. are bonuses.