i'm really glad that i can use my own modem. In germany every ISP is by law required to accept self brought modems. They can't force you to use their often shitty hardware. My current modem/router is up for 3 months without a single interruption to my connection.
I recently moved ISP, partly because of cost, but also because they offered a great home router as part of their bundle. The installer could not utilise any of the existing wiring in my house, had to be all drilled a second time...
Conversely, my last ISP used some awful Nokia modem that barely supported any kind of routing or customisation and I picked them specifically because it was a rental and the fibre wiring had already been done.
It's fairly common for ISP's in Australia to also give you a choice of BYOD or buying one of theirs. Usually you pay outright for the modem, however, so its yours to keep. That said, this is changing with the national fibre roll-out. But with ADSL being the de-facto choice, BYOD makes sense.