Hello, I use a ST350 with a Sonosax SX-R4 for two years now mainly for ambiences.
First of all, you should read some articles from Daniel Courville to well understand ambisonic http://www.radio.uqam.ca/ambisonic/
The ST350 microphone (Format B) is a all in one mic, very easy to use and manipulate but a little bit heavy. The preamp deliver 4 sources (WXYZ) to your recorder. With ST350 preamp you can monitoring a MS signal and some recorder like Sound Device, use their own software decoder.
The beauty of ambisonic is the decoding process ! When you decode the 4 trks in protools, you can "re-arrange" your recording. I mean you can rotate, zoom, use different pattern etc etc.
Other great thing is the fold down.5.1 to MS to Mono.
The microphone sound really great but a little beat noisy when you rec room tone or low level ambiences. You can of course filter the 4 channel in post production.
Protools plug in is fine, but set it up in a session is not convenient.
Nuendo or Pyramix is more flexible for that I think.
Other major deal is monitoring Ambisonic (like discret surround) in Soundminer. Need to open several rewire instance in protools and insert a plug for decoding in SM.
A new online library open with mainly ambisonic material, great site and great people.
Otherwise, if someone need some samples, feel free to email me.
PS : do you like my fabulous french accent ? ;-)