This isn't a new idea - ukulele players have been using fishing line on their instruments for years, and are always looking for something to make their instrument sound "just a bit better." However, for your individual player, buying even 25 yards of string to put strings on just a few instruments doesn't make a whole lot of financial sense, unless you're already using that string to fish with. However, if you have a classroom set of instruments, it's perfect! There's a pretty extensive discussion on this on Barry Maz's GotAUkulele site. (Great resource to poke around at, and lots of fun rants and reviews!)
There's enough to say about this for its own page, so see more about using fishing line for strings on my String Comparison Page.
I found myself in the situation this year where I did in fact have to pinch every penny, and so I figured, "If I suggest it to others I should probably follow my own advice." And since I had spools from my own tests (pun intended), I went ahead and restrung nearly all of my ukes with fishing line. (I keep a couple of "test" ukes different - linear tuning, different types, etc.)
It's working great for the most part, except for one really unanticipated issue. For some reason, the 40# test that I used for the fourth string is tending to "fray" on some of the ukuleles. I've had to replace 3 of the 4th strings out of my 30 ukes that are strung with it. I'd be curious if others find this issue. It's only that specific string, so it makes me wonder if there wasn't a flaw in this particular spool...Something to be aware of, in either case. Not that big of deal to replace the one string, but if I'm recommending it, you should know of this possibility!