Got the eTrex 30 today. Some quick observations:
* Compared to Edge 305, it's thicker but only slightly wider & longer. It weighs 144 g including batteries.
* Easy to match with the sensors and to set up heart rate and cadence display.
* Not compatible with the Edge handlebar mount. Needs a different mount ($10).
* Unlike the Edge, it does not calculate calories.
* When it's connected to the computer, it presents itself as a USB drive, and recorded tracks appear as GPX files on the drive. GPX files do include heart rate and cadence fields. Unfortunately, Strava uploader does not recognize those. So, if you try a direct upload (device -> Strava), heart rate & cadence are lost. I've managed to upload a track without losing data by going through Garmin Connect: upload the GPX to Garmin Connect, download back as TCX, upload to Strava. Some kind of GPX to TCX converter software might work too. It's annoying, but it works.
I think I'll make a longer write-up after a few hundred miles. Looks like a neat device and a viable alternative to Edge.