I cannot speak form experiance as I am about to start my IMC, but the reason I am starting it is that I am planning to do ATPL's distance learning and hour build alongside the ATPL's and my day job. And from experience of the last 18 months of holding my PPL the chance of building circa 75-100 hours in a year or so without an IMC in the UK is quite slim if you are restricted by day jobs or other commitments. So I am primarily planning the IMC in order to facilitate flying when I otherwise wouldn't be able to.
wether its useful for the CPL or IR? I don't know but the discipline of reasonably accurate and instrument flying must be at least a lttle help.