I've also been with Billy for over 5 years now. I agree that the Live Advanced series and Get Ripped were awesome for cardio.
The newer series are different, for sure. Ultimate boot camp is by far the hardest workout I've ever done - but the rest of that boot camp series was pretty unimpressive.
I tried Amped, and then shelved it becuase I didn't like working with the Amplifier. The moves seemed awkward. Recently, I've tried the series again without the Amplifier, and I think I really like it.
I did full Throttle this morning sans Amp and got one hell of a workout. There are parts where I'll add in some moves to keep my heartrate up (jumping jacks, front kicks, etc...), but not that many. I'm more a fan of interval training rather than straight cardio - and that is what it turns out to be.
Very little kicking in this series, mostly punching. (and the Amp DOES work for abs in really cool ways, if you choose to work with it)
I'll do the fat burning DVDs with the amp, but they will not substitue for lifting, as I think it's more for toning.
Right now, I'm using this as a once/week - MAYBE 2x/week workout. I have the full series as well as the add on packages (the 90 minute cardio...).
Hope that's helpful!