Several days ago I came across this interesting article describing how to use Kamailio as carrier grade Least Cost Routing engine and FreeSWITCH as SBC, it is almost one year old, but very well maintained.
First it shows how easy is to integrate both applications to solve demands. Then, although some features offered by those applications overlap, their main target differ, therefore they complete rather than compete.
I am using FreeSwitch pretty much for any new b2bua and voice related application I have to add to my SIP servicing solutions, but still when comes to SIP signaling handling, Kamailio is the king, no mater is about SIP packets mangling, registrar and user location, load balancing or least cost routing.
Overall, it is important to value the blending of open source applications, no matter is Asterisk, FreeSwitch or Kamailio, to build outstanding services and solution. I do not believe in "one size fits all" and most troubles reported out there to each of those projects are due to misuage - trying to do everything with one application, even for functionalities they were not designed for.