The most accurate comparison may be better made after finishing both games, but that doesn't mean only people who have finished the games would be interested in the comparison. Reviews can be fairly subjective, and use a lot of relative descriptions. A topic comparing games (or listing favourite games, etc) can give you a better idea of what to expect, especially if one or more people involved tend to hold the same opinion as you for other games. If I generally agree with someone for games I have played, I'm more likely to buy or check out a demo for one that they like (which I've done so for several games mentioned in the RPG Chat forum or in various topics discussing DD and BD).
If a game is boring or repetitive and you quit half way through, that doesn't mean you can not compare it to other games. It may get better, but you can certainly form an opinion based on what you played. Likewise if a game is amazing and fun but you are not done yet, you can still compare it to other games. Maybe the story falls flat at the end or it gets too easy/hard which may lower your overall opinion, but generally speaking it is usually clear from the context if someone has finished a game or not.
