I use the HPA STM 12X3 short term medical. That is their target, and it is what they do well.
I have actually had better relations with the Celtic reps, here in Ga, than Somarco. However, I was up front with them about their pricing, and got no argument.
If you really want to work with a company that wants to mess with you, just call the Evil Empire (Blue Sh*t, Blue Shield). They're as sweet as punch, and they'll NEVER try to take business away from you. Noooo.
Back to your point. Celtic's STM probably is not worse/better than Assurant's or Evil Empire's. I would tend to go to the one's who specialize in it, HPA.
They took my lithium boy, my guy with the numbness in the foot, and are taking my lady who had brain surgery in July of '05. I have their link on my web site, for people to sign up on-line. I get my commissions on time, and they have a great web site (www.hpa-inc.com).