The Wii U release of Mass Effect 3 comes months later than other platforms. While it comes with a few extras--an extended interactive comic and an exclusive new weapon--the focus for BioWare's port was GamePad implementation. While there are some novel ideas, I found the new features added by the Wii U controller didn't add much to the overall experience.
Visually, the Wii U version is pretty much the exact same game you've seen on the PS3 and Xbox 360. Don't expect higher-fidelity visuals or snazzy new special effects. You will be able to play the game on the TV or entirely on the GamePad--the latter is an interesting option, as seeing a game like Mass Effect 3 run "on a handheld" still holds some novelty. Doing so will provide an experience most similar to the other console versions, as none of the GamePad-specific features can be accessed in this mode.
When playing on the TV, the GamePad features a rotating map, showing the position of your squad, enemies, and objectives. It's a bit of information overload, and not really accessible unless you're already secured in cover. However, once you have a moment to process the info, you'll be able to command your squad, tapping routes for them to take and special moves to utilize.
Shepard's abilities are also mapped to the GamePad, letting you bypass holding the shoulder button to access your skill wheel. You can customize which abilities are mapped to the virtual buttons on the screen, giving you "quick" access to those custom actions. Unfortunately, this works better in theory than in reality. Taking your eyes off the action to look down at the GamePad feels cumbersome--and in harder difficulties, would likely mean the death of your character as it gets pummeled by enemy fire. The GamePad's odd size also makes it difficult to quickly access these buttons. Letting go of your right hand in order to press a button in the middle of the large controller doesn't feel fluid as simply hitting X on a controller, for example.
I actually preferred playing exclusively on the GamePad, as it was less confusing than the TV hybrid play. However, I felt as though there was a tiny bit of latency. My response time felt slightly slower than when playing on the TV--makes sense, given the Wii U GamePad is receiving a wireless video feed. Also, the game obviously loses some fidelity on the GamePad's low resolution screen.
Whereas the late PS3 release of Mass Effect 2 was enhanced with so much content that it felt like a "Game of the Year edition" of sorts, BioWare wasn't willing to comment on what (if any) other bonus content the Wii U version would have. While the GamePad features are novel, they do little to make the Wii U version feel like the "definitive" version--something a late-arriving port should attempt to do.
Mass Effect 3 will be available on Wii U... whenever the Wii U is out.