Trixbox is a good solid build of Asterisk and easy to administer.
Fonality can offer support on the platform if you get stuck or have issues.
The world of enterprise PBX's will, at least at the core, ALL be SIP centered very soon. The mass of the market is already there. My experience with NEC is that they are very late to the game, don't offer a cost advantage over Avaya or Cisco, and are 10x more difficult to work on.
Depending on what your needs really are I would recommend a full re-evaul of the system and look at all comers. Shortel actually has some really good product offerings for a great price. Avaya is actually very price competative for base systems but you will find more premium charges for advanced features and licenses...but at the right scale, you definitely get what you pay for.
Cisco's network synergies are very attractive if you want to have a simplified network/telco architecture.
it's all based on your company's needs and ability to mitigate risk on any given platform.
I have worked with freeswitch and asterisk on custom builds and had them up and workin in a short amount of time.
there is no such thing as a free lunch. you pay for asterisk in terms of man-hours, development cycle, and potential downtime.