Hydra9 does a great job of reviewing this game! The game looks good, especially if you max out all the settings in the configuration file. It is (mostly) stable and bug free (except for the vehicle bug). As you might expect the ending is a let-down. The final boss-fight is poorly designed. The last few levels revert to a boring corridor crawl. The crazy dialogue is entertaining. How can you not enjoy a game whose bad guy is named "Fritt Monkey"? The game features still art "flashbacks" that seem to belong to a different game. A fun and disposable shooter which could have been much better, but is still fun!

It's not a bad idea to design a fast-paced, Doom-style shooter (It worked brilliantly for Painkiller) but Rat Hunter doesn't quite hit the mark. The level design is pretty bland. There are no setpieces and no secret areas - It's just sparsely decorated corridors and rooms. The engine is fast and seems capable but the level architecture is very simple.
The one thing that makes Rat Hunter special (though it's hardly a unique feature) is its use of physics. Specifically, the game allows you to pick up almost any object (even heavy crates) and carry it around or throw it.
It took me a while to realize how integral this was to the gameplay, but once I mapped 'pick stuff up' to the right mouse button, things changed. I started using it all the time, and consequently, started having a lot more fun.
When I got into the flow of the game, it was great. Pick up a crate and hold it in front of you as a bullet shield. Grab an explosive barrel and toss it into a group of enemies, then shoot it to blow them all up.
The game is definitely fun, but it took me about half of its 20 levels before I started to 'get' it.
Even then, I had to make a few changes. Initially, my character's speed seemed a bit low, so I edited a file and sped him up. It was only a 20% increase but it made a big improvement (You can download my tweaks patch if you like). Also, while there's nothing technically bad about the game's metal soundtrack, it lacks variety and I got sick of it, so I ended up replacing the OGG files with some from the Quake II soundtrack - This improved the atmosphere no end, and seemed to fit the game much better.
A few more points:
* The English translation is really poor and the voice acting is unintentionally hilarious. The story is almost nonsensical and your arch nemesis is called Fritt Monkey. Yes... Fritt Monkey.
* There are a couple of AI bugs. If you start the driving level and see cars driving away from you instead of attacking, just quicksave and reload.
* Strangely, almost all the screenshots on the internet are taken from the first two levels of the game. For a bit more variety, check the screenshots I posted on MobyGames).
* Rat Hunter is my first purchase from GamersGate and I'm really pleased that it has no DRM! There is a Starforce 'protect.dll,' but it is never used by the game.
To sum up: Rat Hunter is an average shooter. There are many better FPSs, but there are also a lot worse. It kept me entertained and when I was running around like a lunatic, blasting enemies with my shotgun and throwing office chairs at people, it really started to shine.