My office was on the pre-launch wait list and we got some of the first XPS 18's Dell shipped. For your average person, with average computer needs, it's actually a really nice solution. First off, this really isn't meant to be used like a typical tablet, and is more of a "lap computer". The foldout legs allow it to work beautifully as a presentation piece (I've been using it to do R&D demos), and when reversed it actually makes a very nice tabletop touchscreen.
Is it the most powerful computer in the world? No. Mine is the i5 with 8Gb and it's performance is about average for a modern desktop computer. You're not going to run the latest games with everything cranked all the way up (the lack of discreet graphics puts an end to that), but my son plays SW:ToR and it averages about 40fps with everything turned up. That's nothing amazing, but it's really not bad either. You have to remember that the XPS 18 isn't meant to compete with powerful desktops...it's an Ultrabook in a tablet form factor, and it delivers Ultrabook level performance. When viewed through that lens, the performance is just fine. On the Windows partition, I've run everything from Office to Visual Studio with no real complaints.
The battery life on mine has been fairly good. From a full charge, it will do about 4.5-5 hours of light duty work (web browsing, etc) with the screen brightness turned down a bit. When my son was playing SW:TOR, he got about two hours out of it with the brightness all the way up. That's not the greatest, but you have to remember that we're talking about an 18" 1080 screen.
The portability is actually better than you would expect as well. You're not going to be walking around using it in your hand like an iPad, but it's very well balanced and much easier to handle than it looks. I purchased the messenger bag style case for mine, and usually carry it around like a laptop. When I'm moving around the room, I just tuck it under my arm, where it feels much lighter than its advertised 5lb weight. The back of the XPS 18 is metal, there's a heavy rubber bumper all the way around, and the "gator glass" screen is slightly flexible, which make it fairly durable. Mine has already taken a few falls without any marks or damage.
There are a couple of things I'm less than thrilled about. The power button is poorly placed and is exceptionally easy to accidentally press by hand. I had to reconfigure it in both Win8 and Xubuntu (yes, it dual boots just fine) to ignore inputs from the power button entirely. The foldout legs are well built and seem like they'll last a while, but Dell's folding mechanism uses a poorly designed magnetic holder. Basically they placed magnets on the back of the legs and then placed the regulatory stickers over the top of them to hold them in place. It took two weeks for the stickers on one to peel loose, after which the leg began flopping out on me. It was an easy fix with a bit of superglue, but it was a disappointing to see them cheap out on such a simple detail. Like others, I'm also disappointed in their choice to use a 5400RPM hard drive over a SSD, or even a 7200. The HDD is probably the biggest performance killer in the design. Finally, I'm irritated that, even after a month of tweaking, I haven't managed to get the touchscreen working in Xubuntu 12 LTS. I don't know what Dell did with the drivers for this thing, but none of the standard Linux touchscreen drivers work at all. Because of that, you can only use Linux on it when it's sitting at its base station with the physical keyboard and mouse. It makes a fine Xubuntu workstation when sitting on the base, but I'd really like to get the touchscreen working on it so I can use it as a tablet.
All in all though, I'm fairly satisfied with it. I'm not going to use it to replace my desktop, but since getting it a month ago I've nearly stopped using my previous tablet (Xoom) and have completely stopped carrying my Ultrabook around. My Apple wielding co-workers have largely reported the same. If Dell would offer this in a 15" version and make it a bit more portable, I think they could give the tablet makers a serious run for their money. It really is THAT good. Having a "real" computer in your hand, instead of a hobbled app-only tablet, simply gives you a lot more flexibility in how you use it.
Oh, and for those who said that they'd like it if it would run Win7...it will run 7 just fine. One of my co-workers is rabidly anti-Win8, and the first thing he did was a Win7 downgrade. It works great for him.