I use a simple five star system for ratings. No half or quarter stars. I don't have rigid guidelines for my final rating, but rather give them based on the ever nebulous realm of 'feel'. I'll give a game five stars because it feels like a five star game to me. That's not to say that I don't try to be consistent, just that I don't follow a strict rubric.
Generally speaking, a five star game is one I consider one of my favorites. It may not be perfect, but I will almost always play it when asked. A four star game is a great game that has just a little something holding it back from being a favorite. I'll enjoy playing it, but it may not be the first game I recommend. A three star game is a good game. Mechanically sound, but perhaps not too exciting. It's fine and I'll play it every once in a while, but they tend not to leave a long lasting impression. A two star game is a game that just isn't for me. It may be thematic or mechanical, but I just don't like it. A one star game is a game that just doesn't work. No one should consider playing it because of gross misdesign.
You might have noticed that I haven't awarded any game one star and few games two stars. There's an easy explanation for that. As I'm starting off here, I've been reviewing games that from my own collection and I tend to only have games in my collection that I enjoy. It's not that I haven't played a one or two star game, just that I tend to gravitate towards games I think I'll like.