Just picked up a Surface Pro 3 from local Best Buy. Will probably end up returning it, but decided to give it a try. Random thoughts.
Bigger screen is nice, Wouldn't mind an iPad of this size and weight.
Endless sideways scrolling (for store, news, etc.) gets tiring.
Gets pretty hot when playing a match-3 game.
TouchCover keyboard latches on really tight. Can't take it off in tight spaces for fear of hitting something from the forces of the cover and the Surface flying apart.
When I first put on the keyboard, it didn't initially recognize it. Had to open and close several times before the keyboard finally registered.
Same thing with the "smart on" function. The first few times I opened the cover, the screen stayed dark and had to touch the Windows "button" for it to wake up. Started waking up properly after opening and closing about a half a dozen times.
It's almost as big as the 13 inch MacBook Air. Anything that I do on the MBA will probably feel comfortable doing on the Surface Pro 3.
Haven't tried the pen yet, but it's just asking to be lost. The loop to attach it to the keyboard cover is a separate piece you have to stick on to the cover yourself, and not sure where to attach it so it won't get in the way.
Overall, doesn't quite feel as solid as the iPad. The iPad with a cover feels pretty sturdy, and I handle it like I would a hard-cover book. Can't put a cover on the Surface because that would cover up the vent. Overall, the Surface feels a bit more delicate than the iPad, especially the movable part like the kickstand.
Haven't given the keyboard a good workout yet, but the keys feel decent. Not as good as the MBA keyboard, but serviceable. The trackpad feels a bit "squeaky." Has this sort of rough, yucky feeling against my fingertip. I know I tend to be more sensitive than average about this -- didn't like the track wheel of the early iPods because of how they felt against my skin. The first track wheel I liked was the one on the iPod nano. In any case, I think most people would be okay with this trackpad, but just not premium like the MBA.
So, my feeling at this point is that if you need a super portable laptop, the Surface could be the device for you. But if what you want is a larger tablet, well I really hope Apple is making a larger iPad!
I mean, the Surface is a good device, and in a world where Apple didn't exist, I might be impressed by it. But compared to the iPad and MacBook, it just doesn't rise to the same level of polish. Oh, and carrying the Surface in a bag feels almost the same as carrying a 13 inch MBA. Especially with the key cover attached, the weight difference between a MBA and Surface isn't that significant. Now, if having the Surface means you can ditch your iPad, that might be a meaningful weight savings. But as a tablet, Windows 8 isn't as smooth as iOS.
So to get back to the thread topic, I don't think Apple needs to respond to the Surface quite yet. I don't think Apple can rest on its laurels, because Microsoft is doing a decent job of catching up, but no, they haven't overtaken Apple yet.