It is extremely useful; every PPL who wants to fly around the UK should do it. It will transform your flying, in particular your ability to plan a flight in advance and then actually go, and will very likely put a stop to the 2-3 months which most PPLs spend not flying at all every winter.
Even though it is not valid (as a whole) outside the UK, it also entitles you to fly VMC on top in France which is quite useful a lot of the time.
The skills are of course useful anywhere, and are potentially an absolute lifesaver. IFR navigation is also the best way to do navigate anyway.
Etc.
Drawbacks: you do need reasonable currency to maintain the skills (no good flying 10 hours/year) and you do need a reasonable plane with the right avionics. In practice this may well mean (depending on what you can get your hands on locally) purchasing outright or buying into a syndicate.