I have been a Spitfire reseller since early 2006 and have purchased multiple Spitfire systems since then - see www.callcentremagic.com/clients.html for a representative listing of some of these sites, their missions, size and connectivity method used.
Initially I was selling Spitfire Pro but ever since the launch of the more robust and fully featured Enterprise Edition my clients have been demanding the newer release, which is written using .Net and Silverlight. I now have sites ranging from just 4 seats up to 50 seats using Enterprise, with a pending order for an initial test 25-seater going into a 5,000 strong call centre, split between Australia and the Philippines - I've been told the executives who were handed the proposal laughed when they compared our pricing to the much higher, forever ongoing hosted rentals they have to pay each month at the moment :-)
Another site of interest has the Spitfire Enterprise server in North Sydney, Australia and 20 telemarketers based in LA, California; when the LA-based agents log on, the dialler immediately calls their extension back, holds that channel open then starts placing calls into Australia. Because the channels are already open, this results in very fast transfers as the dialler simply has to conference the channels together once it has detected a 'human'.
Using OPC's superb Support and technical teams, we have integrated with multiple CRMs (MS-CRM, SalesForce, ACT! etc) as well as a variety of PBX platforms such as Avaya, TrixBox and Panasonic systems. In fact, I've yet to find a CRM or PBX we can't talk to...
So, in summary I'd have to say that I wouldn't like my livelihood to depend on any other system, the Spitfire has proved itself again and again with different clients, sites, missions and technology - I couldn't endorse it enough for mission-critical calling.
Ease of use