Since this topic was brought up in comments I figured I'd start off a discussion on the topic.
The kind of strings used on my 12-string guitars (2 of them) hasn't really be thought about much. I have an Ovation Balladeer 12-string (6751) and a Vantage 12-string. I don't play them as much as the 6-strings so that adds to my not changing the strings very often.
I had a bridge issue with the Vantage a couple years ago that led to my getting the Ovation. The bridge was reattached but I'm not that pleased with the work the guy did. I want to be redone, this time by someone who really knows what their doing.
Anyway, strings. The Vantage still has the strings from when it was repaired. They were D'Addario's. Not sure of the weight. The Ovation still has the original strings from the factory.
I do have a set of D'Addario EXP38 Lights that I will be putting on the Ovation soon. I've pretty much convinced myself that I want coated strings on all my guitars. Does anyone else feel the same way?
What other brands, sizes of 12-string sets does anyone else use? Why? Is the tone good? Bright? Dull? Etc...
Does anyone use certain kinds of strings based on the material a guitar is made of? Why?