My DS did this but only to AS level. He did it as his fifth AS. He really enjoyed it. It goes nicely with maths and further maths. He found it comparatively easy and could self teach a lot of it.
There was one student on his course who had thought he was going to be studying an IT type subject.
It is useful for applying for some Comp Sci Uni courses as it shows an interest in the subject but all (or nearly all) Comp Sci uni courses teach everything from scratch and do not expect any specific Comp Sci prior knowledge. Here is some info on Comp Sci degrees and A level requirements and choices.
Comp Sci is basically maths and if a student is a good mathematician then, I imagine, they will be Ok at Comp Sci.
It is not considered a soft A'level. This document produced by the Russel Group Uni's is useful.