1. Depends on your needs, I have one and barely use it but mainly because I usually run my Pis headless. But my friend has one that he uses all the time (he built some sort of audio streaming box with it).
4. I have a few on these for about a year and a half now. my first project with it was as a wifi jamming device that would detect my roommates connecting to my wifi and immediately send de-auth messages to disconnect them every few seconds . It was very fun to screw with them at the press of a button when they didn't want to pay for their share of the internet bill. I also have about 7 or so that I use at work, where we have turned them into automated testing machines that perform overnight tests on products before their release and have become invaluable to us.