When I had Active Home and the CM11A, It was rock solid and never gave me a bit of trouble. The CM15A that came with Active Home Pro is all right, but there are issues with it from some users. The big problem that arose with the CM11A was the fact that as 10 users moved to WindowsXP, alot had COM port problems with the CM11A and had to upgrade to AHP to go to the USB CM15A unit.
My CM15A has not given me any trouble since I got it last July. If you are running AH under Windows XP with no problems and don't need the AHP add-ons, stay with it.
If your X10 system is basic (lamp & appliance modules, etc), AHP will work.