If you record the show from your local NBC affiliate on the day/time it aired, you can ffwd or skip ads or other things. The recording also won't expire unless you have things set up to delete the show when the hard drive is full.
If you record it to your genie DVR from the NBC On Demand free service, not only will it be in crap SD and have an expiration date, you won't be able to ffwd through the few ads/promos they inject into the show or skip segments of the show you might have previously watched.
BIG advantage to just recording your NBC shows from your local NBC affiliate rather than just watch it from NBC On Demand.