What JimLad says is true, but I'll repeat an oft-stated proposal of mine.
There are Treasury agreed Grade Descriptions (not job descriptions) for each CS grade. I suspect JimLad is a Grade C1. In old money, that (to me) is a Professional & Technology Officer Grade 1 or Senior Professional & Technology Officer. In the admin world it was a Senior Executive Officer. For an SPTO, the Grade Description required you to have the proven ability (not the potential) to manage 600 staff. And to be project manager of (a) a major programme or (b) scores of "minor" projects (bearing in mind the two require completely different, but equally demanding skills). "Experience" in the grade was deemed to be, for example, successful delivery of at least 100 "minor" projects (A ruling accepted by Trades Unions in the 90s).
Now, apply those criteria to everyone in DE&S today. How many C1s, or in fact ANYONE at a more senior grade, come remotely near meeting them? Got the picture? So why are they at that grade? That, in a nutshell, is why Servicemen have a generally poor opinion of CS.
But, who is to blame? Not the individual CS. Yes, there are wasters but there always will be. What if the RAF brass suddenly announced that (a) Promotion criteria is being ditched and (b) All pilots are being made redundant but you must carry on flying using anyone you can find. Effectively, that's what has happened in the CS.