Someone recently polled the members of the LIBREF-L listserv for our advice about a second master's degree. When the answers started coming through, I was gratified to see my opinion mirrored back to me again and again. The simple answer is this: It all depends on what you want to do with your career.
Back when I was thinking about what to do for my second master's degree, I narrowed it down to two choices: business administration or education. There are a lot of other fields that interest me, but I wanted my second master's to serve multiple purposes. Both an MBA and an MAEd can serve as a subject master's, but both can also help a librarian do his or her job better. I finally settled on education because one of the things about my job about which I am most passionate is teaching. I'm fascinated by good teachers; I read pedagogical and epistemological research for fun; but most importantly, I love helping students get students to the "Aha!" moment.
That's what works for me, an instruction librarian at a small liberal arts college. What works for you will probably be different. Some people get a subject master's first and discover a love of librarianship through that process. Others get a Master of Public Administration (MPA), and that's a fantastic idea for public librarians. Still others decide not to get a second master's degree - it might be required for some academic librarian positions, but not all.
Again, it all comes down to where you see yourself in five to ten years. On the surface, that is probably the corniest piece of advice I've given yet, but if you give it a chance and really let your imagination go, it just might help. (I know it helped me.)
What about you: do you have a second master's? Why? Why not? If you do, how did you pick?