Yay! First ride of the season, didn't quite get up to 90 but here's my first impression:
When I first put the windscreen on, it was coming into contact with red painted portions circled here:
And nearly making contact elsewhere. I could readily anticipate the rubbing to wear on the paint, so I tweaked the screen up a bit so it wasn't touching (but allowed a half-inch gap between windscreen and headlight cowl). With it like this, yeah, it vibrates: At idle, and definitely at lower speeds (30-45 mph) where the bike is vibe-y and you feel road imperfections. At highway speeds (55+), there's little to no (discernible) shake. The amount of vibration at lower speeds is noticeable, and annoying, so I will be trying to figure out a solution to tweak the screen back down to its original position, with something cushioning between the screen and the cowl at the two points indicated. (It's not on a flat surface. Maybe a type of clear silicone caulk.)
In terms of its wind deflecting utility... Hard to be sure given I didn't do any immediate with/without testing (yet), but it feels like there's a little pocket of deflected air -- up to about my upper chest. (I'm 6', but shorter inseam/tall trunk). It is obviously too small for any rider to properly create a slipscreen, but buffeting *seemed* improved, and this little bit of decreased pressure on my lower torso seemed good? Put another way: Buffeting from vehicles/wind gusts was a thing I noticed acutely on the CB300F last season. On this ride today (not windy, a little breezy), the only time I really noticed buffeting was while passing a tractor-trailer on the highway at 65+.
I like the look slightly less than w/o. If I was buying new, I would have gotten the dark tint version, for aesthetics and given that I'm never looking *through* the windscreen on the CB300F. For $40 used, it seems like a good purchase, but I'll do some better testing later on to gauge whether I'd pay new. There are some other users with this item / threads about it.