A couple months back, the people at elgato sent me a turbo.264 to test out. It's a USB-based hardware encoder for converting any video file to AppleTV or iPod format and comes with software that controls the device. It's a small black plastic stick not much larger than a keychain thumb drive and their site boasts large speed improvements over other conversion methods.
On my Mac Pro desktop (quad core, 2.16Ghz), my results were mixed. Comparing against the best AppleTV/iPod/iPhone converter I know, VisualHub, I did not see any speed improvements when converting an identical file using both the turbo.264 and VisualHub (with VisualHub set to the highest quality setting for AppleTV output). The software application that comes with the turbo.264 doesn't offer any quality settings beyond the output (AppleTV/iPod high quality/iPod low quality) and shows preview keyframes. I assume if I was using an older powerbook or slower macbook I might see some of the advertised speed differences.
On the positive side, the video quality on output files was great. I encoded several identical source files in both VisualHub and turbo.264 and even with VisualHub's settings on the highest "go nuts!" quality, video playback on my AppleTV was smoother and sharper with the turbo.264 output.
So in the end, I didn't see any speed increases, but video quality was noticeably better when using the turbo.264 over a software-only solution like VisualHub. Is the turbo.264 worth the ~$100 price versus a $23 piece of shareware? Possibly, if you're using a laptop mac or older G4 hardware, but for me the quality difference isn't enough to justify the price difference.