About stability and user base, I cannot give you any facts or numbers. I just know that, among others, people from Telenor Research and Innovation (Norway), Orange Labs (France), T-Systems Deutsche Telekom (Germany), or Tata Consultancy Services (India) were actively using the fosstrak EPCIS project and that stability was never an issue on the mailing list - as far as I can remember.
But I agree with Dave, there is barely any active community left for supporting fosstrak. There are currently no plans that I'm aware of for upgrading it to EPCIS 1.2. Of course, the fosstrak project remains open for any contributions - even from large multi-nationals :-)