I've had the chance to fly with COPA numerous times. From my experience, their service is good. When flying from MIA to PTY, I usually prefer to fly with COPA rather than American, Iberia or AeroContinente (especially since they got their 700's).
Tocumen International is a pretty decent airport for Latin American standards. You defenitely do not want to overnight there, since nothing will be open. Traffic is pretty hectic early in the morning & in the evening. A lot of their flights arrive roughly at the same time & turnaround pretty quickly - thus creating their "Hub".
I have heard nightmare stories of Copa as well, but I can say that about almost any airline. I personally have not had any bad experiences with them, so let me know how yours is. BTW, where are you going? If you want more details, feel free to drop me an e-mail.