In my town only the intercity ones used to have them, but these days all the city buses have them too. I don't mind using the seatbelt, but their design isn't optimal - they're hard to put on, particularly if you're wearing a large jacket. A lot of fumbling is required.
In Germany at least it depends on the type of bus. Long distance / travel busses (Google translates it to "coach") are required to have seatbelts, regular "city busses" don't.
As a general rule, buses that go on motorways (ie intercity, mostly) tend to have them; others usually don't.
The first few rows of seats sometimes have those, but not all the rows after that