scallaghan,
This is purely theoretical, as I haven't even started my IR yet. However, so far:
* I have found it mildly useful for P1 hour building as going IMC for short periods is preferable to turning back (with certain provisos!)
* As I need 250hrs TT there is room for 15hrs of extra tuition mid hour building phase. Sharpens one up a little too (IMHO).
* I think provides a better understanding of what is involved with the IR. Gives a chance to cogitate and ruminate on it. My instructor knew I intended to go for the IR eventually so demonstrated a 200' DH and kept me on the ball as far as tolerances are concerned. I think I have a decent appreciation of what to expect now.
Last but not least, it is great fun!
Cheers,
HWD.