I would recommend a desktop PC that you build yourself if you want max performance for your buck, if you want a laptop, please remember that you get Ultrabook's because you need basic functionality (i.e. non gaming) with maximum battery life.
If you want to game there are some really solid 17" laptops that have decent battery life.
15" have the greatest range and also easier to lug around than a 17" ultrabooks are good for internet/word processing i'd only get a 17" if you're always on the move when working and you need something with muscle during your down tim
I'm been looking at the Yoga 2 Pro myself for the past week. Of the new Haswell Ultrabooks coming out, it has the best value by far. The ability to go into a tablet form is pretty nice too. However, there's an issue right now with poor color reproduction. Yellow, in particular, is very bad:
There's a temporary fix where a powersaving feature is turned off which makes the backlight brighter and the yellow looks better. But this causes a significant reduction in battery life. I don't know if this is something that can be fixed by a software or firmware update.
i think the 3200x1800 display is the reason for this issue. It's not your regular RGB panel, where each pixel is comprised of of red, green, and blue subpixels. This one is an RGBW panel, where each pixel only has two subpixels, either red and green or blue and white. The subpixel density is less than if it was RGB, and I think this is the cause of the color issue. The Samsung Ativ Book 9 Plus uses the same display, and AFAIK it has a similar issue but to a lesser extent. Samsung makes the panel used in both, so its possible they're keeping the good ones for themselves...
I'm still hanging back and waiting to see what happens. The Yoga 2 Pro is hard to beat in value, but its issues are hard to overlook. My other choices are the Ativ Book 9 Plus or the Asus Zenbook UX301 and UX302. These are much more expensive for the same specs, but their build quality, especially Asus's, are top notch. I'm leaning more and more towards the Zenbooks at this point. Their Gorilla Glass 3 covered body should keep them looking pristine for years. The UX301's 2560x1440 IGZO display looks amazing. I wish the Yoga 2 Pro used it instead of the faux 3200x1800 one. Tough choices.