I've got one and think it helps reduce the turbulence and loud noise a bit.
Some folks have expressed some concerns: after a rain its possible for water to collect under the deflector and if one opened the sunroof then water could come in, and also there was discussion of the small mounting hardware on deflectors in general can wear the paint down in the area where the deflector attaches.
So as with all things YMMV but I think its worthwhile...