I personally have mixed feelings about Resident Evil 5. I played through and beat it at least a dozen times both solo and with friends, and while the game is actually pretty good, it just did not feel like it lived up to the Resident Evil name.
Again, this is all my own opinion, but I felt that the game was ridiculously easy. All of the other RE titles that I played (RE1, 2, and 3) actually proved somewhat difficult for me between all the puzzles to be solved, all the keys to be hunted down, and all the zombies to fight back. Resident Evil 5 maybe had one puzzle in the entire game (when you play with the mirrors later in the game that 1-shot you if you cross the beams). The Las Plagas (spelling?) also don't pose much of a threat, since they are easy to take out and dodge their attacks. There also seems to be from what I remember of it an easy-mode method to each boss in the game which makes them easily taken out within a couple minutes (minus a couple bosses).
Also, the partner you get really is nothing but dead weight that holds you down throughout the whole game, but if you played with a friend, I suppose it would make the game even easier. I would go as far to say that having a partner increases the challenge instead of being there to assist you like the game intended. Like mentioned already, she burns through any and all ammo you give her, provided she has the weapon to load it with. I personally gave her the handgun throughout the whole game during my first play through and did great.
Don't get me wrong about RE5 though, it really is an amazing game, I just don't feel that it provided enough challenge like the previous games did. Every time you beat the game, it just becomes even easier, and after beating it multiple times, even the hardest difficulty becomes trivial if you activate the extras (infinite ammo rocket launcher, magnum, shotgun, etc.). I personally activate the infinite ammo when playing solo on the harder mode, because it seems that Sheva can't help but dive bomb into a pack of zombies whenever she gets the chance.
Besides all of that though, the graphics are amazing, the enemy players are interesting (zombies toting rocket launchers? awesome.), the cutscenes were also well done and the replay value of this game is through the roof. This game for me did not get old too quickly, since trying to achieve S ranking in every stage can actually be fun. The stages are short enough (and you can actually stage select in this game) where you actually want to go back and try it again to see if you can do better and not get burnt out.
It has been about a month since I played this game, so if I have any information that is inaccurate, feel free to correct me.