For me, using analog tuners in my HTPC for the last 5 or 6 years and now switching the HD-PVR I am in love with my HTPC all over again.
I rarely ever watch SD shows on cable and I never even use the guide below the 200-numbered channels (200 and up is HD) on my cable box. Being able to schedule recordings for commercial-free HD movies and play them anywhere in my house AND sync them to my Zune is amazing.
The only benefit I see to having the Adaptec dual tuner is for the composite video input if I ever need to capture video from an external source like a VCR which I have only ever done once before.
I too am hoping for a "real" driver for the HD-PVR so that it is natively recognized in Media Center and I will never be completely happy having to use an IR blaster to change channels (there is no Firewire port on my cable box). The only way around this is with CableCard or some other authentication to allow my HTPC to decode encrypted premium channels which doesn't exist yet.
To be honest, in my living room I use my dual-tuner cable co HD PVR because of the WAF (GAF); she would hate the delay in changing channels with the IR blaster and the bugs aren't completely worked out with the DVBLink drivers. I sometimes have to restart the service on my PC to get a TV signal once it is lost. Right now I use the HD-PVR with a dedicated cable box connected to my office PC and use it almost exclusively for recording and rarely use the Live TV in Media Center on my HTPC and extenders.