Technically you are no longer forced to use them but the IPSs will try to make your life as difficult as they can if you want to use your own router while often discounting the rent for their AVM routers to $0 (meaning it everyone pais for them with their base subscriber fee even if you opt out of getting one).
Not my experience. With o2, the sales guy on the phone even recommended buying a fritz box when I told him I don't want to rent their "o2 homebox" (which is not made by avm and has a terrible reputation) for 2€ a month.
With Kabel Deutschland the process was also painless. I disconnected their box which they lend to you for free (also not avm), connected the fritzbox I bought on ebay and ended up on some form of captive portal where I had to enter my customer information and boom I was online. I've yet to encounter an isp which tries to force you to use a fritzbox. But then again I wouldn't mind since they are reliable and stable. :)
