Wow. A 5 second boot to usable, 5 second shut down, 64MB ISO. What's not to like? I have it sitting in a VM with as low as 256MB of RAM and 5MB of video memory, and it boots up quick and goes down quick. Sure, I've got a decent processor, but still, I can't imagine this thing taking more than 10 seconds to boot up, even on an Atom or ancient processor.
The thing that I find nice about xPUD is that it's got the winning combination: it's extremely light to start out with, but also customizable. It starts out with just Firefox, xterm, and mplayer, but can add things like codecs, Skype, Pidgin, or even OpenOffice. Of course, this will add to the size of xPUD, but there's the beauty of it: it's your choice. Sure, there's not a ton of applications available (at least, officially), but it has enough to function on a simple level: web browsing, media playing, an office suite, IM, etc. And that's what it is: simple.
So there's that, plus the ability for it to be installed in Windows (C:\xPUD) and dual boots alongside Windows. That's like Wubi on steriods. (Is it? Well, something like that.) No partitions for you to worry about, but then, you also have to install GRUB. Of course you can also download an image and set up a traditional install, or even boot from the ISO in GRUB. In any case, you'll have no shortage if ways to use xPUD however you want.
There's one last thing I should mention: the absolute latest version ATTOTP is 0.9.5, but I highly recommend 0.9.2. The former is fairly new and tests out a few new features while the latter is more stable (in my experience, at least). I've yet to install it anywhere, but if I bring my netbook back to life, xPUD will probably be its latest reincarnation.
I would suggest checking it out if you own a netbook or are just curious about "the quickest way to the cloud."