I did IR first
Having weighed up various pros and cons and talked to a number of contacts from airline pilots to other students, I plumped for doing my IR before CPL.
I can happily report that I found it to be no particular issue, though I should point out that I had done a multi rating previously, so the aircraft/ increased speed wasn't all new. I had an IMC rating when I started, but little instrument time (about 20h).
I would say that if you are a competent, confident PPL, who has found an IMC and complex type flying reasonably straight-forward, IR first is worth thinking about. If you haven't done at least single complex or IMC training, I agree with previous posts that say you will find the learning curve a bit steep, given the time and other pressures during the IR.
I currently in the middle of my 15 hour CPL course, and am finding it quite relaxed after the IR flying, though it's definitely not to be taken lightly and is challenging in its own way. The 'head in cockpit' syndrome is showing itself now and again, but a good instructor will sort that out (and is in my case!)
Money saved? About £1000 overall, but I've chosen to spend it on getting a multi CPL (because I like twin flying mainly).
I think that with this question, as with a lot of others in training, you need to have a good understanding of how you work and what your overall aim is. After that, pay your money and take your choice.
If anyone wants more info/ details, PM me...