About the sound, it depends on what sounds you like. I wouldn't recommend it for fat bass sounds (although it can be done but is a little tricky as it requires the right feedback/drive settings), but it's awesome for plucky sounds, arps, dubtech chords, that kind of stuff. The multimode filter is pretty nice, but I'm not really fond of the lowpass filter.
The FX and sequencer make the biggest difference. You can even sequence the FX section on a per-step basis for crazy sound fx. Routing the tempest thru the inputs is also very nice, you can use the FX of the A4 on the tempest.
The OS is very stable, but it does have some strange things. For instance, it has amplitude modulation, but most of the time I don't hear any difference when I switch it on. The same goes for the notch filter, barely noticable. And the filesystem is a bit of a pain when working on multiple projects. It basically consists of a bunch of patterns grouped in banks. There is no such thing as a project in the A4
The sync and midi department is super tight. Much tighter then the tempest to be honest. Please note, the sequencer notes do not output midi, but turning the knobs do send CC's. So you can't use it to sequence external gear. I think they did this on purpouse to sell more octatracks