I have an Onix 400. The only problem I have with it is related to LOC downloads. The Waypoint format is signed Int LAT small int ALT signed Int LONG. I can not find any software that can read a LOC (xml ish) and convert it. For whatever reason GEOCACHE.com only supports GARMIN as a direct download. It would be nice if you could choose your GPS type in your profile and download to it. I'm sure the hardware vendors would have no issue supplying the format.
As far as a good GPS go. It is good. I do some extreem ATV / camping and it is nice to have the XM weather. GPS does offer models that have the same functions but for double the price. The only issue I did have was when I did a firmware upgrade. It went South on me. I called the help desk, sent it in and it was replaced with a brand new one NIB. No questions ask. The hardware is good but the PC software is poor. I guess the answer is it depends. If all you are going to do is GeoCache. I would stick with a 120$ Garmin.
Good Luck,
Rick