If you are not currently getting 33.6kbps connections across your phone line, you will not be getting 56kbps connections with the new modems. ISDN is works exactly as advertised. ISDN costs somewhat more than two normal residential lines, but less than two business phone lines. Unlike a regular phone you do not get unlimited calling for this, you get a total of 200 outbound calling hours per month. This is normally more than sufficient for your Internet access needs and some outbound voice calls.
Inbound calls are unlimited.
Brooks Fiber will be offering ISDN services later this year, and have indicated their pricing will be aggressive. A normal modem takes 30-40 seconds to dial the phone, wait while it rings, negotiate with the modem at the other end, and start to exchange data. If you pick it up your TA will keep you connected to the Net using just the other channel. When you hang up, the TA will automatically start to use that B-channel also for your Net connection again. This is ideal for having a fax machine connected to for example.