I've heard several... (PE member names posted)
jhollander had his 'BMR-3L' with the 3.5". They sounded great with the right tuning and really doug deeply for a 3.5", however there was a deep and narrow response aberration in its response due to the type of driver it is. It did not seem to be audible. The mechanical damping was really impressive that it did not shoot the membrane across the room.
The following all used the same Tectonic BMR as the BMR-monitor;
4thtry built his 'Totally Flat', and those were just fantastic. He paired it with a small AMT and the Dayton LW5 woofer and won his class at MWAF that year. He also built the 'Plumbers Delight', which won the Nanotech compy for InDIYana and the integration on those was just spectacular.
paulk built his 'Balmora' 3-ways, and they did not impress me. It might be that he used the minimum volume 'required' for the mid, or the fact that I don't like the SB19 in general either, but they were just bland by comparison to a lot of his other projects.
I think it really comes down to the supporting drivers and finesse of the designer. The 2" mid as it stands is not the most resolving in detail, but it tends to be pretty smooth and has great off-axis measurements and responses.