I picked up a SP4 this week. My initial impression was that it was great, but with crappy battery life. I use it mainly for art and it works well. It doesn't feel quite as nice as an Intuos but drawing on-screen is a bit easier and natural feeling. So it's a bit of a trade off. There appears to be some kind of texture on the pen so it doesn't feel slippery on the screen, but it's still not quite as nice of a feel as the Intuos. I miss the configurable buttons on the pen and the quick keys, but overall it's still great for sketching though. I got the i5/8GB version and that is more than enough power for me.
The battery life was the big downside though, the first few days I was only getting 4-5 hours of use out of it under normal use. A bit of research and I ended up doing a registry tweak and turning off Connected Standby and Turbo Boost, which has made a big difference. I'm getting 7-8 hours now.
MS needs to fix the issues with Connected Standby and Turbo Boost, I really shouldn't have had to mess with those to get the advertised battery life. But other than that it's a great device.