I've been using my Surface Pro 4 (Core i5, 256 GB SSD, 8 GB RAM) for the last two weeks, having previously used a Surface Pro 2 for two years, and I think I'm ready to give a brief opinion on it (although there's apparently a huge Win10 update that happens tomorrow but I don't think I even need it to give my recommendation). At this point, I find that everything works the way it should except for an occasional Intel display driver crash while using the Edge browser that it recovers from seamlessly. I initially had a problem with the battery draining during sleep but the update on Nov. 2 seems to have fixed that. My keyboard port also died that same day so I got an exchange done at the Microsoft store.
Assuming that you have a fully functioning unit (which I think I have at this point - not sure how rare this is

), everything is excellent. Battery life is great at 6-8 hours with low-level use, 7-9 with battery saver on. Battery use during sleep is now down to 0.5-1% per hour without doing any tweaks (I've never enabled Hello and I still have WiFi during sleep turned on). I haven't had to do any workarounds or registry edits and, other than the Intel driver issue, I haven't had to mess around with any drivers. OS and app stability seems pretty good, although I'm not pushing it all that hard, mostly just Office 2013, Adobe CS5, Edge, Facebook, Calendar, Mail, OneNote, Visual Studio Community 2015, and Steam open at any given time.
The screen is gorgeous, and I don't notice any bleed during regular use. I'm enjoying the thinner & lighter yet larger screen form factor (compared to the SP2). Pen feels a lot better, both in the hand and writing on the screen, compared to the SP2's cheap plastic pen, particularly due to the textured nib that gives it some resistance while gliding on the screen. The tablet doesn't get that warm unless I'm doing something that constantly requires CPU/GPU and, even then, it's not uncomfortable to touch. The fan, however, is about as loud as it's ever been which might annoy some in an otherwise silent room. Video game performance is about what you'd expect from the Intel 520 vs. the 4400: you gain about 30-40% increased performance which will basically let you run at a higher resolution (1440x900, 16:10 for minimal black bars) with the same settings (ie. lowest-to-low). So far, I've tried Diablo 3, Marvel Heroes, and Gauntlet, and they all run quite well for a tablet.
The Type 4 keyboard is so much better than the Type 2: the keys have a lot more resistance for a more positive feel while the increased spacing between keys makes it feel more like a full-sized keyboard. I actually use the trackpad now that it's not garbage and my fingers can actually slide easily over the larger glass surface.
For anyone on a Surface Pro 1 or 2, this is an easy recommendation: the SP4 has marked improvements on almost every aspect that justifies upgrading if you have the money. This may not be such a big upgrade if you already have an SP3 since you already have the slimmer form factor, although the reduced temperatures & throttling might be of interest to you.
For someone who has never owned a Surface Pro, I highly recommend it if you feel that you want an exceptionally light and portable laptop that can do everything that a larger ultrabook can and you feel that you would also like a tablet to consume media off of (such as during your public transit commute, on those tiny seat tables on airplanes, lying in bed, or removing the keyboard to make space for your food while eating alone). For me, the portability was the main selling point while still being a full Windows 10 computer where the pen allowed for full mouse-like functionality compared to other Windows tablets. There is nothing quite like playing XCOM or Civilization 5 while standing on a moving subway train or Diablo 3 tethered to my phone while sitting on a long bus ride.