This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
For receiving, small is ugly. The bigger the antenna (all else
equal) the better. Of course, all else is never equal, but
these fancy, expensive mini antennas tend to be awful. Some
compensate for their small receiving structure with a small
antenna signal amplifier. However, the quality of that
amplifier is often no better than the quality of the amplifier
in your tuner or receiver, so the antenna just gives you a
stronger signal, complete with stronger noise.
All of that said, some compact FM antennas can work better than
a simple dipole in some situations. Some have an internal
amplifier, which helps with weak signals if the input stage in
your receiver is poor. Some are directional. Some aren't. If
possible, be sure that whatever you buy can be returned for a
refund if it doesn't work out well for you.