I usually love the game before I purchase it, or I wouldn't have taken the risk in the first place. (The exceptions here being games within the Humble Bundles, where I'm more likely to just go for it for the sake of charity, or Steam sales, where games can be under $3 regularly.)
If I end up buying a bigger title which I expect to love and do not love it, the franchise quickly becomes dead to me. (CoD, HALO, Gears) Likewise, if I end up receiving a free copy of a game that was not on my radar and it turns out to be great, I will appreciate it more than a game which I expected to be good and confirmed my suspicions. (Portal, TF2)
If I end up buying a bigger title which I expect to love and do not love it, the franchise quickly becomes dead to me. (CoD, HALO, Gears) Likewise, if I end up receiving a free copy of a game that was not on my radar and it turns out to be great, I will appreciate it more than a game which I expected to be good and confirmed my suspicions. (Portal, TF2)