As someone who owns a workstation PC, an i7 laptop, an iPad 3, iPhone, PSV and 3DS, I have to say as a general purpose device, the tablet sees the most non-production work related use.
There's no reason to turn on the workstation unless I need to render or use apps like motion builder or play Windows games (seldom).
Most of my Photoshop and video editing work is done on the laptop although for a lot of the common minor functions for both photo and video tasks, you don't need to open up Photoshop, Premiere or After Effects. I'm finding that for minor stuff, there are apps on iOS that accomplish the same tasks.
For reading documents, neither of the game handhelds will do. Neither read PDFs, although the PSV is certainly capable if SCE paid Adobe for the license and reader. That said, avoid buying anything that doesn't currently do what you need or want it to under the rationale that "it may come later."
90% of my web browsing, is now done on the tablet. I don't need to do e-mail or read the news or web browse on the laptop. I feel like a complete idiot when I find myself web browsing on the workstation because you don't need a big box burning 300 watts per hour on idle for that. Flash dependent sites can run on the iPad using Puffin, but I almost never use Puffin, which shows Flash is hardly the deal buster it's often made out to be. I have to admit that Hulu is currently my most accessed website that I can't access on the iPad and even that could be remedied with a Hulu Plus subscription.
If you plan to do lot of heavy typing, there are bluetooth keyboards, but I'm typing all this via touch screen. It's not ideal for someone who can touch type over 60wpm and it is slower, but it's not impractical either.
I will say that the PSV browser is usable, albeit a little slow on redraw, but the touch panel makes it as easy to input text as any smartphone. Even easier than the iPhone because of the larger screen. I think if I had to depend on the PSV for web based e-mail, I could get by, but it's far from optimal. E-mail apps to access all accounts at a touch are multiple times better. I don't see this happening for the PSV unless Sony makes a Vita smartphone.
It has Skype, Twitter, Facebook, covering a lot of commonly used smartphone ground, but it is definitely not what I'd consider to be a replacement for one. It goes without saying that the iPad has multiple times more functionality in apps.
The 3DS is for games. The low resolution makes the web browser barely usable, it doesn't read downloadable documents even if it had a file system to store them, etc. It's real simple: buy one for the games and the fun little apps Nintendo has included, but don't try to justify buying one for anything but the games.
In short, the 3DS is for playing games. You want to play them; buy one. That's what it's for.
PSV: limited games obviously. It just came out. As a general purpose device, it is acceptable with some restrictions. Not a smartphone replacement, but pretty good web browsing device, great media playback device, and excellent gaming device.
IPad: pricey if that's an issue, but it replaces most common functions that typical users turn on a laptop for. All untethered from an AC adapter. The always on, instant awake from sleep is something that can generally only be matched by ultra books with great power management like the MacBook Air. The tablet also covers a lot of ground that the PC does not due to the different software ecosystem.
