It has enough postives to be a good game, and worth playing, but also enough negatives to keep it far from being a classic.
If you have played RE4, you will find the gameplay very familiar. The controls (stand still whilst shooting, run away, turn around, repeat) remain pretty much the same. While this doesn't stop it being fun, it does make it feel very dated and clumsy compared to games like Dead Space, which showed that you can have fluid shooting-while-moving controls in a survival horror game.
A lot of the game design feels shamelessy recycled from RE4 - enemies, settings, gas cylinders against a wall to push over and shoot when an ememy is following you etc. etc. , even a lot of the music - you really feel like you're just playing RE4 with upgraded graphics but weaker gameplay and design a lot of the time.
The single worst feature of the game is the inventory system - it is an absolute nightmare - easily the worst inventory system in the RE series - possible the worst in any modern game I can think of - it's bad enough having such a badly designed clumsy inventory system - but to also not allow you to pause the game while you access it, is just a recipe for making you want to throw your controller at the screen - you will frequently die because you are surrounded by enemies & accidentally select the wrong item, or can't pick up some ammo for your empty weapon cos your inventory is full so you have to clumsily pass items to your partner to free up a slot - all using a terrible interface while enemies are still attacking you - I'd estimate that during my 1st playthrough of the game, more than 1/2 of my deaths were caused by the inventory system.
I really can't emphasise strongly enough how much enjoyment the appalling inventory system saps from the game - I can't imagine why they ever released it without fixing that one huge issue.
The addition of a partner throughout the entire game is a mixed blessing - it makes the combat more varied - but it takes away the atmosphere of feeling alone trapped in a scary environment - and her AI is patchy at best.
In RE4, the ememies felt a bit like zombies (even though they weren't technically) - but here the enemies are more human than ever, firing guns at you, riding motorbikes. Apart from the parastites bursting out of them, most of the time you might as well be fighting ordinary humans. Although despite the fact that they can run, jump, drive, use complex machinery - as soon as they get within 10 feet of you, they helpfully return to shambling slowly around so you can shoot them.
One particular annoyance I have - is the guys with chainsaws - there are far more of them in this game than RE4 - but unfortunately, fighting them is really tedious - unlike RE4 where battling a chainsaw guy was quite scary but challenging and enjoyable - here it's just not the same - they seem to be able to take a ridiculous amount of damage before going into "stunned" mode where you can melee them or take time to reload or move to a safe distance, plus the badly designed inventory can make switching to grenades or other weapons mid-fight a nightmare - but worst of all, you might be carefully keeping a safe distance, only for your AI partner to go too close and get decapitated and make you have to do the whole section all over again - in RE4 the sound of a revving chainsaw was the signal of a tense and exciting battle - in RE5 the sound makes your heart sink with a feeling of "Oh god not another one."
Combine that with the lazy level design - eg. in the oil-refinery - turn a switch, and a chainsaw guy appears and attacks - go through the hassle of beating him, try to go through the gate to the next area, and another chainsaw guy appears - beat him, carry on and within a couple of minutes, a 3rd chainsaw guy attacks, this time with the added annoyance of 2 friendly characters in the room who can get killed and force you to redo the section - depressing when the designers were so short of ideas they make you fight the same unenjoyable boss 3 times within a few minutes.
Another major criticism, is simply that the level designs aren't very good - it's just nowhere near as much fun to play as it would have been with more effort put into the level design. This is especially true of the boss-fights - they loook spectactular, but gampeplay-wise they are all long-winded and repetitive - none of them come close to being as much fun as any of the RE4 boss fights.
The online co-op play works well, and the replay value is very much like RE4 - being able to go through it again with upgraded weapons, things to collect - in fact it's probably a lot more fun on the 2nd play through - with upgraded weapons and once you've had time to get used to the inventory system from hell.
There is a distinct feeling that this was developed as a 2-player co-op game, with the single player mode being an extra mode thrown in at the last minute without time to fully playtest and fine-tune it. It's as if they knew the controls and abysmal inventory system would make it impossible to get through so much intense combat on your own, but instead of fixing those issues, they just threw in an AI assistant to give you extra some firepower, but sadly didn't have time to give her very good AI.
I don't think anybody could call this a terrible game - people giving it 1 star on here are being over the top (probably out of anger - an emotion that you will experience often playing this game - so I can understand how they feel) - but it is a real anti-climax after the brilliant RE4 - such weak level and boss-fight design, no real attempt to improve the game-engine and controls beyond improved graphics, sub-standard AI for a partner who accompanies you throughout the entire game, and then having what stands out as maybe the worst inventory system of the last decade of gaming, and then taking the inexplicable decision to make you battle against the inventory system in real-time while enemies still attack you.
If someone had never played RE4 (especially the Wii version with it's enhanced controls) and RE5 - I think I'd have to recommend RE4 to them as the vastly better single player game - that's not nostalgia talking, I've been playing through both games simultaneously to compare them.
If they are going to do a RE6, I hope they have a total re-think - this game is really pushing the boundaries of how far they can get away with it using unnecessarily dated contols & recycling old ideas - plus just too much lazy and downright bad design.
Capcom have already announced that the 2 producers who were brought in to create this game, have been dropped and won't be working any possible sequel - it's easy to understand that decision.