I've been using a usa-28x with my Studio 4 for years - ever since I went from my old G3 desktop to my first MDD. I have never had a serious problem with that setup. It emulates having a printer and a modem port perfectly, allowing me to send MIDI notes down one "port" and MIDI time code down the "other". The only issue I've ever had developed recently when once in a blue moon the unit will hang the OS9 startup unless I unplug the USB plug until after extensions are loaded. It seems to be a standard extension conflict of some kind but it also resolves itself… a few times unplugging and I'll eventually forget to and lo and behold, it starts up without a problem. It then starts fine for weeks and weeks - long enough that I've never been annoyed enough to really make an effort to go after the cause.
What IS annoying is the incomprehensible instructions that come with it. It has a control panel with pop-up help that's little or no help and a lot of settings that are just best left alone. If something does need changing, it's a trial and error process unless you're very familiar with old serial protocol. Fortunately, it seems to like my Studio 4 fine with the default settings. The defaults are probably fine until you try using some rare stylus tablet or such with it. Good interfaces like Opcode Studios do a good job of looking like printers and modems to the system I guess and just always work.
As I've said before (Chris) I've used this setup for years and years and it works flawlwssly. Also, the fact that you can actually still buy a new USA from Tripp-Lite and they're cheap enough to even have a spare makes it a no-brainer! I can't testify to using it with a laptop, only my MDD, but I can't imagine why it would work any differently with any machine natively booting the old Mac OS.