If I played a game for 20 minutes, but had read reviews for it, I would lean on the reviews as well. The people who wrote those reviews have (hopefully) played much further than 20 minutes in, and can tell me what to expect. This is where I look for facts in the review: how long is it? does it continue adding more plant and zombie types? Does it have any other game modes?
Yea, this is part of why I like 5 star scoring systems.
Giant Bomb's rating system explanation:
5 stars: While we don't believe any game is perfect, we recommend this game without reservation.
4 stars: Still very good and easy to recommend, though it doesn't quite live up to its full potential.
3 stars: The halfway point. An inherent appreciation of this game's specific gameplay style, characters, subject matter, and so on may play as big a role in your enjoyment as the actual quality of the game.
2 stars: This game's problems outweigh its good qualities.
1 star: This game will make you wish you had died in a fire moments before turning it on.
There are games they gave 2 stars to, that I own and enjoyed, but agree with the rating. Aliens vs. Predator comes to mind. According to those descriptions, an FPS fan might thoroughly enjoy a 3 star FPS game. Basically there's only ONE rating for shit games, instead of 70 percentage levels for them.