Pros: game pad, small footprint, actually has power, more 1080p games, Nintendo exclusives, great versions of 3rd party titles, Miiverse, eShop
Cons: limited 3rd party support, no online multiplayer for certain Nintendo games, no Blu-ray playback, needs more classic and indie games on the eShop
First, the Wii U plays GAMES and it plays them well. Disc and downloaded games all load just as fast as any other system I own. Plus, I have yet to encounter any buggy issues with my 3rd party games. I honestly can't say the same about some of the 3rd party games I've played on the 360. The Wii U supports full 1080p resolution and 60 fps. I realize that not every game runs at that on the Wii U, but the system can definitely support it. It actually has more 1080p games than any other system right now. Your mileage may vary depending on the publisher. However, the Wii U supports full HD and looks great on my Samsung LED LCD. I've played great first and third party games on the Wii U. Many of the standouts have been Nintendo titles. However, Mass Effect 3 and Arkham City really shine on the Wii U compared to the 360 versions, at least in my opinion. I'm disappointed at the lack of 3rd part support overall though. I'm sure Nintendo is owed some blame here, but it's also up to the publishers to put out some good content. Companies can't expect to port older games to a new system and have everyone falling over themselves to pick them up. I've read conflicting things on Nintendo’s power under the console hood. However, as far as I can tell, the Wii U is more powerful than the PS3 and Xbox 360. I'm sure people will argue on both sides of this, but I can honestly say I'm not disappointed in the look of games on my Wii U. I really wish more 3rd party companies would jump on board. I certainly buy the Wii U version of a game when possible.
The game pad on the Wii U comes off as a gimmick to some at first. However, it can really be an amazing asset. There are Mario games that require you to blow into the mic to move platforms or touch the game pad's touchscreen to complete certain actions. That's all fine, but I am not too concerned about that. Where the game pad shines to me is in having easy and fast access to your game menus without stopping play. It's really handy and it genuinely does improve the experience of playing a game with a map or lots of menu items to deal with. Also, I REALLY use the off screen play from the game pad when I'm playing less menu/map-oriented games. It's another great feature. I can be anywhere in my living room or kitchen playing a game while the TV is on something else. My wife can play Skyrim, or we can watch TV without giving up my game time or putting the system in another room.
The Wii U can stream online content like Netflix, Hulu Plus, etc... I don't often use that, but it's there and it works without issue for me. Nintendo TVii is another feature that would probably be more useful for someone that has cable and watches lots of channels. Personally, I don't use it much. It was quite easy to setup and I'll occasionally switch channels and adjust volume with the game pad. It does not play Blu-ray movies. I know you are supposed to buy a system to play games, but that would have been a nice touch. I've already got a BD player, so it isn't a big deal to me.
All Wii remotes and Wii games are supported by the Wii U and it has a sensor bar for above the TV. Additionally, there is on integrated into the game pad too. I don't own a Wii, but that might be of note to some migrating over.
Also, the eShop is great for picking up old games and new indie titles. I've had lots of fun with indie games like Runner 2, Mighty Switch Force 2, etc... I'm hoping Nintendo keeps encouraging the indie relationship.
Setup on the Wii was pretty easy. I have a Samsung LED LCD TV and small surround system. No issues for me. It took just a bit longer than other systems because I needed to calibrate the sensor bar and setup Nintendo TVii. I think some people had an issue with downloading an update when the console first launched. That wasn't an issue for me though. I needed no updates to get going. Pretty painless as far as setups go.
Again, the game pad is great! I love being able to use it to navigate the menus by touch. The controller is surprisingly light and comfortable. I also have the pro controller that is more traditionally shaped. Honestly, I prefer the game pad because it is a bit wider. The pro controller is very comfortable though. Again, the control setup for the Wii U is even more comfortable for me than my 360 or PS3. The menu system is a series of tiles that can be accessed by touch or with a controller. I usually do it by touch on the game pad. No issues for me here. It's nice having almost everything laid out in front of you and not paging through lots of sub-menus. If you run out of space on the main screen, new tiles are automatically added to another page though.
I've partially addressed this already. My games load as quickly or faster than my other systems. The system is much quieter than my Xbox 360 and a little quieter than the PS3. It seems to run cooler as well, but I've never done a scientific measurement. All of the games I've played have had very sharp 1080p graphics. Again, no issues there and I prefer the Wii U versions of games now whenever possible. I haven't noted any lag online or from the TV to the game pad. The game pad's lack of lag is astonishing. It really does make the experience flawless. There aren't many online games for the Wii U that take advantage of multiplayer, so I don't feel I'm qualified to speak on that end.
The game pad really makes things special on the Wii U. I can navigate quickly through game menus, utilize it for novel mechanics in games like Super Mario 3D world, or seamlessly transition to off screen play and free up my TV for something else. Having the Wii U gives me the pros of a console and some of the pros from a handheld system. Range is limited on the game pad though. I couldn't go into my basement or on the complete other side of my house to play.
Sound is immersive on the games I've played so far. Again, the game pad also has sound. For some games, it is a mirror image of what the TV outputs. In others, you'll hear sound just coming from the game pad that isn't played on TV. This isn't a bug. It is intentional. An example would be in Arkham City, radio surveillance can be heard on the game pad like it is a radio while the action on the TV is normal game sound.
Streaming playback has been great although I rarely use it. You can watch Netflix/Hulu solely on the game pad if you want too. The console does not play Blu-ray movies or DVD's, so that could be a deal breaker if you don't already have a DVD or BD player and want something to do it all.
I really found this to be an underrated system. I initially purchased it so I could play some Mario and Zelda. However, it's quickly become my favorite over the past 5 months. I'd like to see a few more 3rd party titles, but Nintendo has so many good 1st party titles and indie games that I've been staying busy after finishing Mass Effect and Arkham City. If you were even remotely considering a Wii U purchase, I would jump on it. It really has been the most pleasant surprise of any system I've purchased since the PS2 or Super Nintendo.