We've had a mascot for our planetarium for over 10 years, and so
ahve some experience in their use as a marketing device.  At first
we used the mascot extensively, starting with a "name the mascot"contest.
 Over 1000 kids entered, and we chose the name SULUNA, after the
sun and moon design for the head.  SULUNA had a regular volunteer
assigned to the costume, and made appearances in parades, festivals,
before civic and church groups, and at the museum for family events.
 We even made a "parade banner" so that SULUNA could march in our
city's Christmas and St Patrick's day parades on a regular basis.
 The volunteer we had developed a "program" and a "personality" which
made the mascot particularly memorable.  However, once the volunteeer
moved on we were never able to recapture the magic the use of the
mascot diminished, and SULUNA became a generic creature.  When our
museum staff was "rightsized" last year, the use of the mascot was
essentially phased out since we didn't really have the staff to accompany
her to events.  In retrospect, I'd say that if you have the time
and staff to develop the program, a mascot can be a valuable addition,
but it needs to be more than just a volunteer "stuffed" into a costume.