Zyxel Prestige 324 router, "Traffic Redirect", anyone used it?

I am looking for a router for my new wireless connection which will
make it easy to use my existing ISDN as a backup.

I've searched around and while looking at the Zyxel Prestige 324 FAQs
etc. it would seem that its "Traffic Redirect" facility is just what I
want. If the WAN connection fails requests are redirected to another
IP address which can be on the local LAN, thus I can leave my ISDN
router connected as at present and point "Traffic Redirect" to its
address.

Has anyone any experience of using this facility and/or do any other
routers offer anything similar?

--
Chris Green