I switched to ID 5 or 6 weeks ago.
They're working off the 3 network which has its issues and blackspots, but I'd previously been with 3 for years, and don't notice any difference in service.
I had initially planned to go prepay, but seemingly you have to go into the carphone warehouse shop to buy credit, so I went bill pay. A really nice feature is you can tweak what's in your bundle, and although bill pay you can set a cap on the max your bill can be and it won't let you run up a bill beyond that amount. My bill is 20, and I'll be able to bring that down once I work out what I use.
I haven't tried tethering, but I don't understand how they can block it.
Edit: just checked, and I can't find anything in my contract about tethering. It's blocked on ID UK, but seemingly not here. When I said I don't understand how they can block it, u meant I don't know enough about it, not that it can't be done (before someone picks me up on that).
In general using mobile to replace broadband, I'd highly recommend you read
@dub_nerd posts here if you haven't already
https://www.askaboutmoney.com/threads/phone-hotspot.205846/