I did it: we now use Asterisk as home 'pbx'. I bought an OpenVox dual ISDN card via Novavox. Great company, Novavox: when it would take about a week to deliver the card they got in touch to make sure I could wait that long.
The choice for dual ISDN was because I still want to keep the outside line via KPN isdn: we are also using budgetphone but it was quite easy to find a phone number that was unreachable via budgetphone but reachable via KPN. And I want to keep using an ISDN set. So one port of the isdn card is configured as 'TE' card (terminal equipment) connected to KPN and one is configured as 'NT' card (network terminator) connected to the ISDN set. Asterisk does all the heavy lifting: outgoing call routing depending on number called and time of day. Incoming call routing, reacting to callerid and advanced answering machine.