Hi - yes, I actually purchased and field tested three beacons during a recent Avalanche training course. Of all the beacons (including Mammut Pulse, and BCA Tracker 2), I was the most impressed with the S1+. (Note: they were all good, and did their job. Read on).
Ease of use is pretty straightforward - open, and you're searching. Closed, and you're sending. If the beacon is open for too long (ie: you were hit by another avalanche while searching for someone), it will automatically revert to Send mode. A small LED flashes to let you know it's turned on in the holster.
I was the most impressed by the S1+ on the "Range Test" (where the instructor walks further and further away with a transmitting beacon, while 12 students read out the distance indicator) - the S1+ was the one of only two beacons in the group to go past 40 meters (the Mammut Pulse was the other one). The S1+ though - it kept reading past 60 meters. This might not seem like much, but if I'm searching for one of my best friends or my wife, I want as much range as I can darn well afford - the small extra cost for a better beacon is suddenly going to be worth it.
The other test that sold me on the S1+ was during multiple-burial drills. Comparing the S1+ and the Mammut Pulse, I personally found the S1+ to have a faster processing speed while locating 3 or more victims.
Downsides - the fine-search is different on the S1+ than some people might be accustomed to. But anyone who has taken an Avy course will quickly and easily adapt to the circle instead of a big "+" symbol.
Which brings me to my last point - if you're reading this and looking for an Avalanche beacon, but haven't yet taken an Avy course - Stop. Spend your money on the Avy course first, then a good beacon second. You'll be informed, and will make an educated purchasing decision as a result.