From the stuff I've read on the 2-pop forums, the Wired MediaPress line of hardware encoders is the best (if cost doesn't matter). Their drivers are still OS 9, but that is supposed to change:
http://www.wiredinc.com/newsletter.html
Cleaner with Charger is great, also expensive, but I hear that Cleaner with Supercharger (the hardware option) is actually slower than apple's MPEG encoder included with DVD Studio Pro. Of this, I would go with the Charger, because it addition to greater encoding control, it also does MPEG-1, which the SuperCharger doesn't do. Also, while Cleaner runs in X, the Charger and SuperCharger do not (and may never).
http://www.discreet.com/index-nf.html
Then there is Heuris, and their software seems to be really good, but slow.
http://www.heuris.com/
And then there is DigiGami, who I know nothing about, except that they were kind enough to include a simple, though limited MPEG-1 encoder with premier 5.
http://www.digigami.com/
I don't know about heuris, but digigami definitely runs in OS X.
Oh yeah, newcomer (to mac) called BitVice. Know the least about this encoder, but the reviews on the versiontracker list seem pretty optimistic (they seem to have a demo version).
http://www.innobits.com
All I've tried are the Cleaner Charger and DigiGami "light" version.
Charger gives you pretty exhaustive control, but it runs classic. (it continues to encode fine the background, though, which it doesn't do in straight OS-9)
There is my box of pennys!