I had 15% Lumar installed all the way around for $125 with a lifetime warranty. (no pics online, yet.) It provides enough shade to make driving comfortable during the day, but at night it does affect my ability to determine rearview mirror distances slightly.

Check your local ordinances.

One good thing to know: Lawsuits here in Oklahoma have demonstrated that there is no reliable way for a police officer to check your tint.

EDITED: The police can not determine what % you are using, but don't try to deny having tint alltogether.