We've had excellent luck deploying ISDN over Centrex whenever NYNEX actually
slips up and loop-qualifies a site :)
Charges run $112.50/month within the same CO, and $172.50 + $9.89/mile
across CO's. This allows us to deploy non-metered ISDN dedicated lines
for an average of $300/month. This is a great price/performance item,
but ISDN isn't available everywhere in RI, though NYNEX's marketing says
it is.
Setting up a client with an OU-U dialled into a 5-BRI board has been
great for us.