I run both a SciAtl 8000 and SA Tivo (on one TV).
Nice to be able to record three shows at the same time ... Also, digital channels recorded via the 8000 have a noticeably better PQ than via Tivo.
Anyway ... if you don't use the 8000 much as a DVR ... then controlling it with Tivo isn't that bad. Make sure to set the "On With Channel Change" setting on the 8000.
If you DO end up using the 8000 much as a DVR though ... it starts to be a bit of a PITA controlling it with the Tivo. I ended up splitting the cable and running the coax straight in to the Tivo. The Tivo now can only record the analog channels ... but ... that's what works best for me.