I was finally able to take a look at the "self aligning" type from Waterway (400-5090). It's still a flat connection, like a regular union, but it has a bevel on the outer edge of the connecting surface that centers the two sides when you pull them together. From what I can tell, I think that this would help get the threads caught a bit easier if the pipes weren't aligned quite right, and would force the sides to center up as it was tightened. For situations where the pipes were really misaligned, the "Swivel" type would be better since these literally allow the pipe to be pointed in a slightly different direction (I believe that the Waterway version -- 400-4481 -- allows up to 18 degrees of tilt.)
I'd still love to hear from anyone with experience in using the swivel type -- i.e. do they seal just as well, etc.?
Larry