I've been using Boom 2 for nearly six months now. I bought it for about $10 for my work MacBook Pro for a simple reason: the volume could just not get loud enough. At the time, I was watching movies at home by connecting the MacBook to my 26" HP screen. With many shows, the volume was just weak. It got old. Then I got hit by some offer via email, looked at Boom, and bought it. I've been pretty happy with Boom ever since. Yes, it cranks the volume notably--especially on the MacBook Pro.
I've been upgrading my work computers. Moved to an iMac 5K. It's speakers crush in terms of quality and volume compared to any other Mac I'v heard, so it did not need Boom. Didn't even try. However, the MacBook Air I have is, well, thin on sound like it's thin on body. Transferred Boom 2 over to the Air. Yes! The quality did improve as did the potential volume increase.
Important point with Boom: you select your Mac model and it adjust the sounds accordingly. This is important to know, especially if you connect your laptop to external devices. For example this past week I came in to my office and plugged my Air into a 26" Mac monitor. The sound was AWFUL. Tinny, hollow--just utter trash. Same result when I plugged in some Bose USB speakers. WTH I asked.
Then I remembered Boom: it adjusts the sound for the specific computer and not speakers. I turned of Boom! and things were great again. Clear, good sound from the speakers, from the monitor.
The app is definitely worth the money: completely improves listening quality and potential volume.