Garmin Forerunner 405 and 910xt: these models (actually the entire family of ANT+ Forerunners) can simply use the internal ANT+ chip. Support is in alpha, seems to be working well for some but not for all of you guys. Details here.
Garmin Forerunner 305: this model requires a USB OTG adapter. I got one on ebay for a couple of euros. Just plug it in, it works like a charm. Several other models falls in the same family.
Garmin Edge 500: this model won't work if you simply plug it in. The Xperia complains about excessive current on the USB port and shuts is down. In practice, I expect this to happen for several other devices as well.
The workaround is simple: I used a powered USB hub. An alternative is to use a USB Y-cable plus a charger.
The connection procedure seems to be critical:
- plug the edge into the powered hub.
- plug the hub into OTG adapter, not connected to the Xperia yet
- plug the OTG adapter into the Xperia
- the edge is mounted, the app is started.
Note that if you do it any other way (first you plug in the hub, then the edge, or you plug in the OTG adapter first) it does not work. I tried the procedure several times.
Overall: the Xperia Ion will probably handle the entire line of Garmin devices supported by the app. Some may require external power. Some requires just an OTG adapter. Some may simply use the internal chip, no cable and no dongle required.
There is no need to root the device, or anything like that. Thanks Sony :-).
I can't vouch for the entire Xperia line, so if you try it out on another model, consider leaving a comment here to help the other guys.