I finally checked out a XBOne controller and my first impression was definitely good.
Build quality is noticeably better than the X360 controllers (including the special editions).
The first major improvement my eyes darted for is the d-pad. It is world's better than the original X360 d-pad but I can't say it's necessarily superior to the more recent transforming d-pad (from some special edition controllers). It is actually a d-pad now as opposed to a flimsy rotary plate but the contact points are clickers, so it kinda feels like each direction is a "mouse click." It works relatively well for menu browsing but I haven't put it through the real test: Street Fighter IV.
The gamepad's overall shape is akin to the wired X360 controller. The battery compartment has been reworked into that slimmer frame. I do appreciate Microsoft's choice in keeping the controller battery operated; you can always choose to get a Play'N'Charge kit if you want to take the "recharge" route.
The analogue sticks have a rugged texture minimizing thumb-slipping. The sticks' sensitivity has been tweaked, it's very similar to the DualShock 4.
The triggers are much wider, though they narrow towards the tip. They have clearly received a great deal of attention, squeezing the triggers is smooth as butter. From the moment I start pressing I only feel the slightest resistance but no other mechanical bits or any plastic squeaking. While I love the DS4, one complaint I had about the triggers is the "cheap plastic feel" and squeaking each time I pull a trigger.
The shoulder buttons LB/RB are technically an improvement but they're not especially great. They're still clickers but they seem a bit more responsive. The only issue I noticed was that I have to be sure I click them at the very edge. If my finger is drawn up too far they might take a bit to mechanically respond.
With regards to connecting the XBOne controller to a PC, it's got the opposite problems of the X360 controller (which could only be connected wirelessly with the receiver), which is manageable for the time being. As we discussed earlier, it can only be connected by wire so at least you don't need to occupy two USB slots like the X360 controller (one for receiver and one for charging). I never understood why the wireless receiver for the X360 controller didn't have an integrated USB slot (like the way Logitech do it for some of their wireless gear).
Anyway, the XBOne controller ($60) seems to make a good replacement for the Wired X360 controller ($40) for Windows but at the price it's a tough sell.