Smiley face EQ on the boost
This is my favorite pedal. It's really a meat and potatoes dinner of a pedal. Yes, maybe some of my boutique'y pedals sound better side by side outside of a musical context, but a Tube Screamer circuit just EQ's your guitar perfectly to "fit" in the music. That cursed bass cut, the mid poke, and that dulling of the top end IS actually the magic dust, so I'm not sure why everyone complains about it. Anyway, the TS side does that. The 9-18v doesn't seem to do much for the TS sound, but 9v maybe feels a bit squishier.
The boost isn't exactly neutral. It has a scooped sound (or hyped bass and highs) that seems to be the exact opposite EQ curve of the Tube Screamer! Hence, if you have an amp you can set at the edge of breakup, goosing it with the boost will give you a full bodied crunch. And then you can throw the TS on to poke out the middle for solos. Works great! I always use the boost after the TS. The 18v setting seems to make the boost a bit strident. I much prefer the 9v setting on this side, too.
In summary, it's the mainstay of my pedal board and I'd buy it again. I highly recommend this if you're in to Tube Screamers because the added boost is EQ'ed perfectly to match the TS side.