Good graphics, awesome music and controls that are easy to understand but hard to master make this a perfect remake.
The remake is from the ground up, graphics, sound and some gameplay elements have all been modernized while keeping in spirit with the old games. The graphics are good there are some nice effects, real-time shadows and explosions but nothing eye-popping, this is an XBLM game after all. The character models overall are pretty undetailed, they serve their purpose but not much more. I think the main effort went into making sure that you could tell which enemy was which by looking at their profile or colors. The bosses however are magnificent large and colorful. Each bossfight is preceded a conversation between Nathan "Rad" Spencer and the boss. These conversations are often humorous and presented in beautiful hand drawn, comic-style images.
The music itself is exceptional. If you like remixes of old Nintendo / C64 tunes you are in for a real treat. The soundtrack is available on iTunes and I think I'm going to buy it because it is really worth it. The sound effects aren't really anything special but serve their purpose.
The main gameplay elements remain the same and so does the main controls. You've got two main buttons to use; one which fires your weapon and one that uses your arm. You can also throw grenades (unsure if this was in the original) and switch between different weapons. This last point is a new feature from the original which only allowed you to take one weapon into each stage.
From what I gather most of the stages remain reminiscent of their classic version but have had some layout changes. This is the same as most of the bosses who mostly remain the same but have had some behavior upgrades or tweaks to their strategy. The bossfights are overall very enjoyable and very memorable, from a construction site robot with lose bolts to a giant assembly machine that locks you into place and attacks you from different directions with attacks you have to repel with your arms. Each boss is absolutely huge and usually the game zooms out to get the entire boss to fit on the screen.
I can't really comment on the co-op and competitive multiplayer since I haven't had a chance to try them yet.
One final point I'd like to make is regarding the controls. Some people seem to degrade this game because of lousy controls. However by doing that it seems like you have missed the point of the entire game. The controls are fairly simple yet it requires practice to learn how to handle them. I can't see how anyone who has played the game longer than an hour can say the controls are bad because that's when you start to learn how to properly play the game. The B button shoots the arm. If you are standing still Rad shoots it diagonal in the direction you are facing. If you press up or left / right you shoot it in that direction. Fairly simple but very hard to master and that's part of the fun.