Communication with Anton before I ordered was very good, he gladly answered all questions I had. The amp+speaker arrived well packaged, installation instructions are very clear. I'd advise to order it with the pre cut and pre soldered wires, saves some hassle :-)
For me, someone with hardly any soldering experience and a soldering iron with a too large tip, installation was kind of a challenge, but in a good way! Had fun, and it worked out. Lifting that pin with a small hobby knife (good tip from the other review) wasn't too hard. Checking afterwards if I had done it right, was much harder, as those pins are so, damn, small.
No problem at all to fit everything in my GBA, absolutely had no issues with that.
Sound volume is just perfect, a little more than you would normally need, and perfectly loud enough for some games I have with pretty low volume. This actually is the sound volume the original GBA should have had.
Sound quality is a bit too harsh for my taste to be honest, too crisp and sharp. After some testing found out the "problem" (more like a personal taste issue I guess) was in the speaker, so placed back the old speaker and am perfectly satisfied now.
Installed the GBAmp3 in my modified gameboy advance with backlit screen and it works like a charm! The sound is crisp and loud enough for a gratifying retro gameplay. Recommended to buy together with wires as they are cut to the correct length by the seller for easy installation, which I think is a nice touch. Really glad that such amplifier for GBA existed! Soldering is moderate difficulty but if newbie like me able to do it, then anyone also can.