I've used the new Bavarian Demon Axon (and an old 3SX), I've got an Axon fitted to a Henseleit TDR.. It's a decent unit, it has a few of it's own little setup 'quirks' that you need to get used to but I found it pretty easy to setup overall. Default settings are very aggressive compared to most FBL. Tail stops for instance are very hard and cyclic response extremely 'direct' which can make it feel twitchy, this is all tunable but the naming of the settings is a little cryptic.
The Axon is a very nice unit overall but for the price I'd rather go for the Spirit Pro (which has a similar layout and size to the Axon/3SX). Tomas (the guy behind Spirit) used to be the developer for Bavarian Demon and he took all of that knowledge with him when he setup Spirit. In my opinion the Spirit is the better unit, plus is significantly cheaper. The Spirit is more advanced especially if you are interested in stability options and want to use telemetry and programming integration. Spirit also has an optional 'Geolink' module that will give you return to home, position hole and a whole bunch of other GPS related options (if you like that sort of thing).