Going to get slated, but...
... I actually think it's a pretty good idea to be honest. I qualify that statement by saying that I'd be much, much happier if I knew exactly how it worked - particularly around prevent LAN access to people who have access to your Wifi.
But really, this won't impact corporates (and if it does then the problem is with your implementation of Wifi rather than Wifi Sense - you should be using 802.11x over PSK's FFS!).
So it's actually going to be for consumers over businesses that will be impacted by it. And if MS are storing the key encrypted at rest and over the wire, as well as it only being shared in a non-visible way to your address book with WAN access only.
To be honest, with the track history and how much is "sensitive" data is stored on cloud services (iCloud, Google, OneDrive) a PSK that's limited to WAN only access and shared with people in your address book only - that you can change / revoke - doesn't bother me that much.
Considering that with Chrome your web-based passwords are sync'd via Google Cloud, and with IE / Edge your credentials are sync'd with OneDrive as the store then my home Wifi PSK seems fairly "safe". Well, safe enough.
Until otherwise stated, at present you need to be in my address book and have physical access to the Wifi signal. Even then it's just WAN access too.
I have used this as an owner of a Lumia 930. My brother has a Lumia 1020, whilst my mum has a 730. Works really well - just seamless... They pop over and turn on their Wifi.... and voila - internet access without the need to shout out keys.
Also have a Linx 10 tablet that I've been running Windows 10 Insider for a few months. Again with this - always have Wifi access without dicking about with keys. Have tested it out with some test scenarios and indeed, there is no LAN access. Haven't investigated why, and as a techie I'd really like to know how this is achieved, but on the surface it seems useful.
Sure, there maybe a potential / possible security issue - but in reality and so far it all seems good. Worth noting that it's not shared with Facebook by default. You have to explicitly give consent for just that one type of sharing.