I have one but I've never used it - I'd love to though.
I think I opened it up one day or checked the service manual and it was full of 741 op-amps (or of equivalent calibre/era). So then I thought... hmmm would that improve or completely destroy the sound?
For those that have no clue what this device is, it plugs directly into the FM tuner's audio signal output (phono output jacks) and then performs its magic to reduce the FM multipath (read: radio signal deflections and reflections from the radio station on the way to your aerial resulting in your tuner receiving multiple delayed copies of the same radio signal) caused noise/ghost signals.
In the Carver tuners that use it, I'd guess this works straight after the FM detector circuit, i.e. having as input the L+R and the L-R signals produced by the detector. This is my own assumption based on the references to the circuit and the invention I've read on the internet. I haven't actually checked the Carver tuners' schematics.
In this device, however, you would lose that sonic benefit (i.e. of directly feeding the device with the L+R (left and right channel sound summed - the mono signal) and the L-R (left and right channel signal difference) signals. Instead, since you can only plug this device at the left and right output of your tuner, by necessity, the device must reconstruct the L+R and L-R signals, do its magic and then re-combine them to produce the cleaned up left and right outputs. In other words, you have an additional two redundant stages in your sonic chain. To make matters worse, these are implemented with 741s (or equivalent).
Now, having written all that (which are mostly speculation), if you have a station with particularly poor signal (poor due to multipath effects that is) that you must absolutely tune in to hear, then, yes, I would say that this device could prove to be very useful.