Remember Resident Evil 4? Remember how it was an amazing, atmospheric game with addictive gameplay and days of replayability? Remember how it was almost the only reason to own a Gamecube, it was just that good? Do you remember the worst part of the game? It was the escort segment of the game - the part featuring Ashley where she'd run around slowing you down. It was the single black mark on an otherwise flawless experience.
Did you ever sit there and think to yourself "Gee Resident Evil 4 could be perfect if they gave Ashley a gun"? I didn't. That train of thought never even approached my brain station. I did think to myself "I wonder if I can shoot Ashley in the god damned face and go back to the US pretending she was already dead when I got there". Ashley's superpower is that she can only be injured by unwell Spanish villagers though, so shooting doesn't help.
Apparently in the Capcom offices the cranial conductor was having an off-day, because they not only decided to make EVERY SINGLE SECOND of Resident Evil 5 an escort mission - they decided the person you're escorting should actually waste what little ammo you have.
Sheva Alomar, your BSAA (Think SWAT for Zombies) team mate and the token black chick (when the game was first announced the thought of Chris shooting Africans was regarded as "racism". A black woman should keep the Polically Correct Police at bay!) is the brand new Ashley in Resident Evil 5 - and she's present the entire game. You share ammo and weapons with her, you share health with her and you share her deaths. If she gets her dumb self killed it's game over for you too. She is in fact the worst thing about Resident Evil 5... And she's also the best.
Because when you replace the dumb as nails, waste your ammo and default to pistol AI with a human player you're left with one of the best action shooters in recent history. It's tense - the decision to stick to inventory management and stop-and-shoot as core game mechanics ensures this. It's graphically pleasing - the gore, the physics and the animations are top notch. Well, it wouldn't be a Resident Evil title if it didn't have appalling facial animations, but characters run and interact well.
In fact RE 5 trumps RE 4 in many ways. The mini-boss fights with the chainsaw guys and the new, just as deadly guys are still just as freaky. The storyline makes more sense - as much sense as a bio-engineered weapon which turns people into semi-sentient zombies with giant whipping, exploding heads can - and it features cameos from some old favourites. It delivers a more detailed background, the pacing is better and the characters are all-around more interesting.
It's shorter, which isn't great, and the replayability is limited (because the odds of you getting a partner to play through the entire game a second time are slim and you're not going to do it with the AI) and the puzzles - already dumbed down for RE4 - are even more obvious in RE5, but it's a better game.
Newcomers to the series might have issues with the almost Metal Gear Solid-esque combination of buttons you need to press to either melee attack or fire a weapon, but you'll pick it up quick enough. The game is well-balanced - easy enough on low difficulties to still provide a challenge while maintaining a certain sense of strategy when you start to hit veteran. There are a few things which go regrettably unexplained in the game, like the shop system, what you're supposed to do with extra guns and how to properly "combine" items.
Really at the end of the day the question you have to ask yourself before you purchase Resident Evil 5 is this - are you on your way down to pick up RE5 alongside a friend or has a friend of yours already picked up a copy of the game? Can the two of you make time to play together the 17 hours from the beginning to the end of this game? Do you and your friend get along well? Do the two of you own the same console?
If you answered "No" to any of these questions, skip Resident Evil 5. The teammate AI is painfully bad and it will go down as one of the biggest mistakes in video gaming history. To take you back to the annoying train analogy from before, Resident Evil 5 is a train where the engineer is retarded and steals all your god damned ammo. Replace that engineer with a human and you'll have fun with the game.