The reason for Reaper's relative popularity is a bit obvious; it costs a fraction of the others' at $60.

If you're starting out on the home recording journey and budget is an issue (and also as a newbie you may be unsure as to whether the hobby is for you) then it's a no-brainer, Reaper is the one.

I tried it a while ago out of curiosity but found it difficult to deal with, which is borne out by a recent Sound On Sound review which concluded it was an excellent product but demanded a lot of user pre-configuration:-
"Users of other sequencers will usually need some time to adjust to the Reaper mind-set. You need to invest some time in making the many user-configurable settings if you’re going to get the best out of it."
It puts a much steeper initial learning curve in front of you, especially if you're coming from another product, but with ample rewards afterwards.
SOS Review
John


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