I bought one of these the day Win 8 came out. It's been a mostly positive experience with a few minor quirks.
The performance is acceptable for what it is. Don't go in expecting a gaming beast, but it's great for surfing the net/watching videos on the couch/in bed and the kind of stuff you'd reasonably be doing on the go.
Battery life is just about 5 hours of video playback off YouTube and the like, or a bit better off the drive. Close to 8 hours of just light web browsing of reddit and such. (Note this in in power saving mode with the screen turned down to a level appropriate for an almost completely dark room) I took it with me while waiting in line for 5 hours for the Hobbit and it was at between 35 and 40% when the movie started tethering it to my phone with wifi. (Actually outlasting the phones battery)
My initial complaint when I got it was how much of the 128GB SSD was allocated to recovery partitions when i got it (the C drive was 64GB, so I had just over 40GB after windows was installed). They've since released a patch on the website that's supposed to increase the C drive to 100GB, but I had already manually re-formatted it before that and freed the entire drive.
The fan isn't horrible, but it's not as quiet as they make it sound. at a distance of 3-4 feet it's almost inaudible during light loads, but using it from a more normal laptop distance you can hear it. And when using it as a tablet from a distance under a foot it is not what I'd call quiet if you are laying in a position that has the vents facing toward you. The biggest sound issue i have is the fan virtually never shut off. Even in power saving mode, which defaults to passive cooling, the only time I've ever gotten the fan's to shut off is when you let the screen turn off, or intermittently in a room temperature probably in the mid 50's F.
There is a case designed for it you can use when in tablet mode that slots over the back to cover the keyboard, but although it exposes the power button, volume rockers, etc. it cover's the USB ports so it's of limited use. Also it feels cheaply made for being $40.
Although I haven't done so myself, from what others have said on the forums it's actually got decent upgradeability for an ultrabook. By just removing the keyboard (double sided tape) you can replace the RAM (single slot, so take out the 4GB to put in an 8GB stick). Or by also removing quite a few screws (but not having to take any actual components out i don't think), you can replace the SSD (mSATA) or even add a second SSD as it comes with two mSATA slots and one is empty.
Though not 1080p, the screen at 1600x900 is better than that of a standard laptop, and the not being 1080p will actually give it a slight advantage trying to run games at native resolution (Though I haven't installed any to try it).
The Keyboard is adequate, though not amazing. I seem to miss the space bar occasionally when typing, but that's just me not a hardware issue. Truthfully I use it in tablet mode much more than anything else.
Most of the mode's work great, but using it propped up with the keyboard down and the screen angled (basically reverse laptop) can be hit or miss on an uneven surface because the default keyboard/trackpad disable point in rotating the hinges seems to be set too late so they often don't disable until it's rotated past 300 degrees, when ideally they would disable at between 180 and 270 at the latest.
Overall I'm very happy with it though for what I wanted, which was basically a bigger tablet running the fill x86 windows that I could use as a laptop when out and about. I have a desktop for gaming/heavy use already, but I use my Yoga nightly as well. It's also great for when I need to do a quick edit in Photoshop or remote in to another PC for work for a couple minutes to do something quick but don't feel like getting up and going to my desktop to do it. My only complaint from a work regard as an ultra-portable is no wired Ethernet, but you can get a USB dongle for that easy enough, which I did. The price ($1000) is a bit higher than your standard Ultrabook of the same specs, but well in line with other machines adding similar dual functionality of tablet use in various ways.
The value of full x86 compatibility in this, as well as the other similar devices soon to come out, makes it a huge step up from your standard tablet in functionality. As much as android tablets are improving in this regard, windows still takes the 'it just works' crown by a large margin. And in that regard, for a device like this Win 8 is great. I'm still town for desktop use between 7 and 8, but for something along these lines 8 is a no brainer.
TLDR: If you go in with reasonable expectations it won't disappoint you. Remember it's a Ultrabook/tablet hybrid not a desktop or even high performance laptop.