The 'Options' tab in the preference pane gives you exactly two options: a checkbox to turn Overscan on and off (either losing a bit of the screen real estate over the edges, or leaving a small black border around the image) and a seemingly-inactive "Best for Video" checkbox that remained a mystery to me while I was testing.
On the audio side, choosing 'Digital Audio Out' from your Sound preference pane will route your output through the adapter. Unfortunately I don't have a full-optical receiver setup -- I think the next test site for this adapter will be David W.'s home theater, to give it a fair shake -- but the audio quality and levels sounded comparable or better than my usual analog link, at least to these amateur ears.
Although I expected that the 720p setting would look best, matching the native resolution of the display, it actually looked rather hard-edged; type and onscreen graphics had slight halos, although that effect was mostly unnoticeable when playing video. I got better results from choosing 1080p (perhaps because it brought the display's own downscaling circuit into play) or the 1344x756 resolution. In either case, the screen detail was good and the display was clear and sharp, without any noticeable lag or bleeding.
I played back a few different content types, including iTunes-purchased HD television shows, DVD movie backups from Handbrake, live HDTV via the Elgato EyeTV tuner, and streaming 480p content from Hulu; they all looked as good as they did on the MBP's internal screen. Audio was synchronized and clear across all sources.
I did a couple of comparisons to see how the Kanex compared to other signal paths or HDMI sources. First, I watched a few minutes of the same episode of Dollhouse both from an iTunes HD purchase and from my Dish Network HD DVR. While the DVR playback seemed a bit more fluid and watchable, the iTunes ep also looked very good, even showing some shadow detail that wasn't visible on the broadcast version.
The next comparison was versus a DisplayPort-to-VGA adapter plus analog audio cable; since my TV sports a VGA connector, it's easy to use it as an external display, although the two-cable solution is not as elegant as a single HDMI. The VGA seemed a trifle sharper and didn't have the overscan/underscan option of the Kanex; it filled the screen correctly at both the 1344x756 and at the 1280x720 resolutions. Again, I couldn't hear a marked difference between the optical and analog audio paths, but that's more a reflection of my speakers than a gloss on the quality of the optical link.
While the EyeTV content looked comparable on the VGA and HDMI paths, there was a very big difference between the two approaches when it came to the iTunes HD television episode. Playing it on the HDMI adapter, as noted above, worked like a charm, but trying to play it over the VGA connector gave me a roadblock: HDCP checks will not let you play protected video out to an analog video destination. With its digital-to-digital pathway, the Kanex (along with other DP to HDMI solutions) has a big leg up here with purchased content.
Given that the Kanex adapter does a yeoman's job of getting the video and audio onto HDMI and over to your TV, is it a worthwhile buy? That same $70 could buy you an audio-free 10-foot mini DisplayPort / HDMI cable... seven times over. With basic no-audio adapters in the $10 range, and Monoprice listing what seems to be the same product as the Kanex for $50, there's a fairly limited target market for this gadget.
For anyone who doesn't mind a bit of cable clutter, it almost certainly makes financial and flexible sense to go with a less-expensive adapter sans audio and route the sound to a receiver or to aux ports on the TV. For those with VGA/RGB inputs and little interest in HDCP-locked content, you may be better off with a VGA cable and an audio ride-along.
If you're one of the AV-aesthetes who would rather have a single HDMI cable snaking across the living room to your laptop instead of a bundle of video and audio cables, you may be happy shelling out for the Kanex. Another point in its favor is that it will bundle your audio signal onto HDMI, making life easier if you don't have the option of auxiliary audio-in with your HDMI ports. Yes, there's a $40 DVI/TOSLINK adapter for HDMI; it requires its own power connection, though, and it's a bunch bigger & uglier than the Kanex. Is sleekness and ease of use worth the extra $30 to you?
Kanex is reporting that the adapter is sold out at the moment, but if and when you decide to give it a try I would double-check the company's return policy (or the third-party vendor's policy, if you can't get it direct) in case you discover that your particular setup doesn't play nicely with the unit.
The vendor provided a review unit for testing. To see TUAW's policy on hardware and software reviews, please visit http://www.tuaw.com/policies