Mine arrived earlier this week, but I didn't have much time to test it. I'm using it on a PI3b+ and it works like a charm. I tried Monkey Island 2, Police Quest 3, Rise of The Dragon, Lotus 3, Star Wars Dark Forces, Dune and Dune 2 and they worked out of the box without need of SoftMPU. I had to use SoftMPU for Monkey Island and Loom though. Now that I remember either Dune or dune 2 gave me some strange error when in MT-32 mode, very weird, out of place sounds played, but I solved by simply switching to CM32L.
SoundFonts work fine too, I find it exciting to think there are so many out there.I tried SC55 and found it good enough, but the XG one I tried was a bit underwhelming for me. Luckily I own an SC-88VL and an MU50.
I had some problem with Timbres of Heaven while playing Grabbag from setup, it started fine but when the synths came in, they were too forward and swallowed most of the other instruments. It was also louder than most other SoundFonts I've tried. But I'm sure this has nothing to do with MP32L itself.
I also had some "Unexp midi STATUS" errors while using your typical MIDI+Game Port cable, but I solved it by using a DB15MIDI and a proper Roland MIDI cable.
All in all, I'm very happy with this little thing, it's versatile and sounds great in my opinion.