Undoubtably the best flying training you will do short of a full IR. Improves the quality and precicision of your flying no end. You really feel on top of the airplane all the time. You can actually use all of the knobs and guages in the aircraft properly!
It gives you a safe way out in poor conditions, rather than trying to skirt around under the murk at dangerous heights climb up, get steady and sort yourself out for a safe approach.
I agree with IOS, IFR navigation is by far the best way to navigate, and when flying VFR it gives you much greater situational awareness, there is no reason for an Instrument pilot ever to be lost.
With a night rating and an IMC it opens up a whole new world!
You need to maintain currency both with flying on the guages and doing approaches to be able to rely on it.
The only limitation being its only valid in the UK, for me a lot of foriegn travel mean an upgrade to IR but if I was just flying in the UK the IMC is perfect.