Different sound fonts can make midis sound better but it can also make some midis sound worse.
Currently I don't use this.
Yeah, the bad part about midi is it depends on the soundcard, and the instrument used in the games they're in, as well as the soundont itself. I've had mostly positive experience with it though.
It's pretty buggy when used with ZC, but if I'm playing for an extended period of time, and I know I'm not gonna be leaving that window, I tend to use it.
Agreed, it's kind of annoying when it changes everything to piano. My usage is about the same as you described. I tried the latest update for it as well, but it tends to only allow one program to use it, so I reverted to the 1.17 one, which opens a new synth for each program that uses it.
From personal experience (and not to continually bring it up, but), the Jurgen_GM_GS_Bank.sf2 is probably my fav to use with ZC, it's got a great set of instruments, and in some cases, with the right midi, it can make a song sound just like the original from one of the zelda games. For example: The Wind Temple from Wind Waker (which is in my project), and alttp's light world track. In a lot of cases it takes changing a couple instruments, but that's something I'm accustomed to doing for projects lol. Can't do that with qst files, but I haven't found any real bad sounding ones, mainly just spots where the instrument doesn't quite fit based on my personal tastes.
And of course, I fall back on my 8-bit sf2 when in doubt, or if I'm playing a qst that has gameboy link, or the original tiles.