If I were using more of the library, I'd definitely switch to upper-left origin, but I prefer lower-left since it makes more sense to me mathematically, and converting my existing lower-left-origin program would be kind of a pain.
At this point, I'm just using Slick for the fonts and asset loading. I was using Slick-Util instead, but it only includes the deprecated TrueTypeFont. I'd also be interested in including the newer Font classes in Slick-Util, and I guess adding such a feature would probably only be useful to that end.
I'll check out LibGDX and see if its font support is as nice as Slick's, and if not, I'll probably just hack a local copy for personal use.