I never go with more than 20%. Anything more looks like someone I'd profile if I were a police officer.
15-20% allows plenty of cabin visibility and still makes the passenger cabin much more tolerable on sunny, hot days.
You can see through it well enough that someone took a hammer to this window when parked on campus in an attempt to steal an iPod left on the seat (note the marks on the window). The Neon's glass won.
