Bear in mind that I've only had a short time to play so far - and I'm at work. And I'm on the first version of the firmware (I can't connect to work WiFi with this device)
1) The UI is pretty snappy. It's different to the old AK Jr, but in a good way. You've seen the YouTube I linked earlier in the thread. Scrolling is good, and you can drag the slider bar up and down for even snappier scrolling. Search button works well for searching for album/artist etc. (but you have to go back to the main screen to use it each time).
2) Yes. Sounds different to my previous AR M20 - cooler, more transparent, more honest I'd say, but I'm not an expert listener by any stretch of the imagination. I also don't have balanced headphones.
3) Pros - lots. Snappy UI, sounds great. Cons - The design makes it difficult to pick up from the desk. The side buttons are a bit too small. If you have an artist with many albums, you have to scroll past all of the songs in each album to get to the lower albums. There's no sorting options. You can't stop the screen from blanking out and you're forced to press the power button on top to wake the screen back up (there's no tap-to-wake).
4) It's a subjective choice. I don't want to buy another DAP for a long time to come, so I reasoned that over time I'd save money on chasing the rainbow. As much as I liked my old AR M20, I hated it crashing every few days (and it having no pause button). I figured that the SP1000 has stood the test of time, so the SE100 should do the same.