Now that I've used it for a bit, here are some quick first impressions as didn't have a chance to see this in a store before.
Previously, I was using a Surface Pro 2 which I recently sold. While I liked that device, the weight and screen size were two negatives for me. The SP3 completely takes away those concerns for me. The screen size is incredible and I love the weight and thinness of this device in my hands. I've heard some reviews mention that the screen is too big for tablet users, but I disagree completely. I'm 6'2" so maybe that skews my opinion, but this works so well in tablet mode that I think I can finally convince my wife to sell her iPad and laptop and move over to a SP3 device. I could even go a bit bigger and still be ok (SP4?).
A couple of things I noticed that may be of interest to others...
There are no Windows Updates yet besides KB2969817 which is a Windows 8.1 update. Any firmware or other related SP3 updates that have been rumored, I assume will either be released tomorrow or at some point soon afterwards.
I had an issue pairing my Surface Pen during the initial setup. I then tried pairing it from within Windows Settings and it only worked when I turned off my nearby laptop which also has Bluetooth enabled.
Feel free to ask any questions and I'll try and get to them between "working" on my SP3 and watching England beat Uraguay!