My company is a major reseller of IP Office & INDeX we are also moving to suport BCM,
On paper the BCM certainly looks to be a capable product.
The IP Office is allso a capable product, there are a few bugs in the latest level of firmware ( which should be resolved withinn the next week), however this is true for almost all systems (anyone who says thier system is bug free is either lying or does not fully know the product!)
If you are migrating toe IP Offie from INDeX the bigest problem you will have is user familiarity.
It uses the same handsets but there are a number of operational differences that can result in the usual " the old system did it this way" type complaints.
for more info on the IPO check the IPO forum, & I would sugest you check the posts here to get a feel for the BCM.
Unles you have some unusual requirements either system should be good for your needs (although the IPO will requre LESS user retraining).
I have tryied to give a fair asesment to your question, but as I have more experience of the IPO & INDeX than the BCM this post should be considered biased