The term 'bustitution' seems to be preferred in other English-speaking countries.
The popular image of a rail replacement experience is waiting in the cold, or wet, or dark, for a bus that might or might not come and staff just shrug when asked. When it finally does arrive it's a clapped out Leyland Olympian with benches barely big enough for schoolchildren, rattles like hell, and has an odd and overpowering musk.
These days it's a decent modern service bus or a coach (always fun seeing a London bus in deepest darkest Sussex) and staff information is usually pretty good. The information boards at unstaffed stations showing where to catch an RRB are usually accurate as well and those maps have been very helpful, especially if you're unfamiliar with the area.
Obviously, an RRB will almost always be inferior to a train in terms of comfort, journey time, and facilities, but it's better than nothing at the end of the day.