Overall I'm very happy with the Vita. Here are the things I like about it:
1. It's comfortable. While the buttons and the analog sticks are a little small, overall it's the most comfortable hand-held device I've ever owned, except maybe the original Gameboy Advance, which doesn't count because the non-backlit screen was so god-awful that the system as a whole was unusable. Speaking of which...
2. The screen. The large OLED screen is a joy to watch. I have an iPhone 4 and an iPad 1 and the Vita (IMHO) trumps them all for screen quality. The resolution is 540p (half of 1080p) but since most PS3 games only run at 720p anyway, the difference is negligible. Try playing Blazblue on a 3DS and then play it on a Vita. The Vita wins hands down.
3. The controls. The fight sequences in Uncharted really give you an immediate sense of how awesome the second analog stick is. I would kill for a PS Vita Bioshock game, or maybe a Batman game or a BioWare RPG. With the updated graphics combined with the dual-analog controls, I think we'll see a lot of franchises that are popular on PS3/360/PC to find their way to the Vita. And I'm really looking forward to it. It's also worth noting that unlike the 3DS, the d-pad on the Vita is located in a sensible, usable spot so for fighting games, for example, it's actually enjoyable to play them.
4. The interface. Gone is the XMB, and the new touch-screen interface has a nice feel to it that, while definitely iOS-inspired, is unique enough to have its own feel.
5. Region-free. Unlike the 3DS, which is completely region-locked, I can buy a Vita game from any region of the world and I don't have to worry about whether or not it will work. And many popular games get an "Asian" release in Taiwan in addition to the Japanese release, which has Chinese and English. So when I got Uncharted with my system, I didn't have to slog through the Japanese and figure out what I'm doing, I just chose "English" as my language and I was all set!
Here are things I don't like.
1. Expensive memory cards. You basically NEED a memory card to use the system properly, and right now the only affordable one is 4GB. I can buy SD memory cards for my 3DS at a fraction of the price for the same capacity, but Sony decided to gouge its customers with expensive proprietary shit. NOT impressed. And it's costing them business too, because there are some older PSP games and DLC I'd consider buying off the PSN store, but I don't want to sacrifice the space on my memory card. Even worse, to prevent casual piracy only one PSN account is allowed per card. So if I want to buy games or DLC from both the Taiwan and the Canada PSN store, I need two memory cards and I have to swap them to go from one account to the other.
2. More of the same. Unlike for example, Super Mario Land for 3DS, which is a unique style of Mario tailor-made for that system, I've yet to see a good Vita game that really NEEDED to be on the Vita. It seems to suffer from the same problem the PSP had, where aside from being portable, the Vita experience and the home console experience aren't fundamentally different. I have Minna no Golf 6 and it's basically the same game as Minna no Golf 5, just portable. Same for Ultimate Marvel vs. Capcom 3, Blazblue Continuum Shift Extend, Rayman Origins, etc.
3. When it's different, it's worse. This might sound like it contradicts my earlier point, but efforts by Sony and developers to make the Vita version of their game feel different or special usually fail miserably. Their answer to uniqueness is to throw in a lot of touchscreen/motion control crap from the iPhone and Wii. In Uncharted you can fortunately turn most of it off or ignore it, but even still I found myself doing things like touchscreen puzzles and holding the Vita's camera up to a light in order to progress in the game. Lame.
Like I said, overall I'm very happy with the Vita. If I could get games like Super Mario Land 3D and Mario Kart 7 for the Vita, then I'd sell my 3DS and not be bothered with it. As it stands, I play my Vita more than any other system I own (I have eleven consoles and six handhelds in addition to my iPhone and iPad) and I'd play it even more if there were more games out for it (SSX on Vita, make it happen!). For me, it's been a fantastic system.