Having replayed the whole Resident Evil series a few months ago, RE5 is a terrible single player game. It can definitely be fun with two people, and even has a few mechanical improvements over RE4 like more complex melee systems and strafing. But it's simply terrible as a single player game.
The level design isn't nearly as interesting, with only a handful of good setpieces. And because your AI partner is so dumb and uses up actual resources they make the game a lot less fun. Not to mention simply completely ruining the mood. The boss fights are a pretty big step down from RE4 too. Most of them rely on one-time gimmicks to finish. RE4 had some fights like this, but for the most part the bosses in that game challenged you to make the best use of the arsenal and skills you'd built up to that point. In RE5 turn on the furnace, or grab the flamethrower, or shoot the rocket, or get on the mounted machine gun. There are only two major bosses in the game I can think of that force you to do otherwise.
The game has a severe lack of pacing, in most areas you're just moving from similar combat section to combat section without any downtime to think. There were some cool ideas though, like being able to walk past the lickers to avoid being detected. But then there were awful things like the gun-toting enemies. That game's combat system was not meant to deal with lots of opponents with firearms. And the story, while unimportant, lacks any of the campy charm of RE4, trying to play itself almost completely serious and failing almost entirely. Only Wesker really exudes the fun camp that the series has pretty much always had. And then there are some other things, like that awful turret segment, something I hate in just about any game. At least it didn't have as many bad vehicle sections as RE6.
Mercenaries was SO good in RE5 though, and Mercenaries reunion was even better. I hate that it never came to PC.