I spoke to my [very picky, audio-wise] friend Carlos yesterday. I asked him if he had ever used the Reflexion Filter and he said he tried it, but returned it because it was too 'colored' when he used it on one of his small diaphragm omni condenser mics. I asked him, "Colored--in what way?"
"Boxy-sounding," was his reply.
I told him that was funny, because the vocal samples I sent him to check out were done with it and I didn't hear even a trace of "boxy"... he agreed and wants to try it again. He thought about it for a moment, then remembered that he had bought the smaller, less expensive version.
Also, I found this in the extensive thread on gearslutz:
"The optimum mic position relative to the filter is to get the capsule lined up with the centre of the filter and to have the mic sitting just within the curve of the filter, leaving plenty of space between it and the filter wall. If you place the mic further into the filter, the sound dries up more, but may change slightly in tonality"
Now that I spent around an hour reading all 8 pages of the gearslutz thread I would say the consensus is:
1. Most people who bought it like it, some like it a lot and use adjectives like "Focus" and "Clarity" when describing their results. Several said it removes negative room ambience in their environment.
2. A few people thought it was colored-sounding. "Nasal" and "Boxy" were the adjectives used. One guy hated it and sold it immediately.
3. Generally it was suggested that it wouldn't be beneficial to use if your room was already well-treated. However, one guy with a well-treated room still preferred using it to not using it.
4. It was suggested that the best sound would be obtained by keeping the mic not too deep in the shell, and that putting it deep in the shell would color the sound.