Freephoneline is beta testing a Mac client, which is nice.
If you do buy the sip settings, you can use any sip client you want, even clients that work on the Mac.
As for the reliability of freephoneline, it is not five 9's reliable, and I doubt it ever will be, because freephoneline does not have control over your Internet connection, your sip device or your router. Because of that, VOIP is not reliable. I consider it pretty good for a VOIP provider though, and the $50 is pretty cheap considering what you get.
Look at cell phones 10 years ago. you had to pay top dollar and you still ended up as a beta tester. I would have killed to have a service as reliable as freephoneline then!

Such is technology, I suppose!