Unfortunately one is comparing two moving targets: FAA IR done in the USA, probably pre-9/11, with JAA IR done here in the UK today. Or, FAA IR done in the UK today, with the problems with FAA examiner availability and a host of other issues, versus a JAA IR done here in the UK today.
Today, based on what I have seen, the JAA IR ground school is probably 5x more study than the FAA IR, but the added hassle of any flight training in the USA changes the picture significantly for many people.
It however remains true that the "best hours credit" route remains
IMCR
FAA IR (full credit given for all IMCR training and all solo instrument time)
JAA IR (credit given for previous ICAO IR training, all but 15hrs in the UK)
The IMCR represents excellent value to a UK PPL who needs a significant mission capability improvement. For UK IFR flying, the full IR adds little beyond that unless one has access to (and the budget for) a much more capable plane than the average rental spamcan.
There are indications of a substantial PPL/IR theory reduction and I have this on good authority, but have to say that this has been "just around the corner" for as long as I have been flying, yet the European IR has got harder at every modification, for as many years as anybody I know has been flying. All attempts at simplification have been smartly crippled before implementation. One has to wait and see. However, I would not do the JAA IR today unless I had an actual need - it isn't likely (IMHO) to get any harder under EASA and it could well get easier.