Anyone have experience with such speakers? What are the advantages and disadvantages?
I've been manufacturing speakers with rear-firing tweeters for about fourteen years, but mine don't look like the Snell. Mine use horns.
One thing a designer can do with a rear-firing tweeter is, make the power response (the summed off-axis energy) smoother. For instance, the Snell's front-firing tweeter will have a pattern which narrows as frequency increases, resulting in a spectral imbalance between the direct sound and the reflection field. A rear-firing tweeter's response can be tailored to compensate for this.
A rear-firing tweeter's contribution will decrease the direct-to-reflected energy ratio, which may or may not be desirable depending on the specifics.
Depending on the reflection path lengths, a rear-firing tweeter will often shift the temporal "center of gravity" of the reflections back in time a little bit, which can affect the spatial qualities.
A rear-firing tweeter tends to increase the sense of spaciousness, but if it is too loud, it will degrade clarity and image precision.
And a rear-firing tweeter (with its associated high-pass filter) redistributes driver cost. So for equal driver cost, less money can be put into the front-firing drivers.
Obviously I think a rear-firing tweeter can be a worthwhile net improvement since I've been doing it for a long time, and obviously I'm in the minority.