hm, forgot to enable mail notifications, sorry for the late reply
FS' wiki states that people knowing asterisk are disadvantaged compared to people coming without any prior background (except of course voip knowledge), and that's quite true: I scratched my head many times trying to understand how FS did things. It doesn't mean that FS is more difficult to configure than asterisk, it's just that people are usually bored re-learning things.
I was a little bit annoyed by the all-XML config style but in the end you'll find out that it's a rather good technical decision as soon as you delve into more complicated setups - and that's where really FS shines. For instance, I had to use a standalone sip proxy because asterisk's sip configuration didn't support multiple external ips (the voip server was NAT'ed in a DMZ, and sip clients could connect through different internet providers, with/without VPN). FS handles that by design and provides a clean separation between sip instances (running each on a different port).
For sure that post doesn't help you much (and since I don't know your voip setup I can't give you specific examples), but yes, I like FS more than asterisk and I didn't have any stability problems. Security seems to be taken into account at the architecture level, and FS is really flexible, so no worry for complicated setups.
WRT integration tools, we had custom scripts/interfaces so we could adapt them to FS (LDAP user management, accounting, ...). If your setup is complicated I'm sure the devs will help.
On the downside, FS doesn't have asterisk's critical mass, so good luck finding an admin who knows FS :(