I have lived in Glendora for the better part of 20 years. Robbery is the most common type of serious crime that occurs here, although in 2013 there were two murders - I know of only one other murder in this town. Crimes like robberies have been on the rise here over the last few years, which, according to the police department, is because of the early release of criminals from California prisons.
That being said, Glendora is a very safe place to live. We often leave our vehicles unlocked and have never had an incident, other than a kid roaming the neighborhood in the middle of the night opening car doors and looking for change in the ash trays.
There are "good" and "better" parts of town. Most people believe that you want to be north of Foothill, and I agree. Below Route 66 you find more multi-family housing, and south of that, you find some trailer parks. These are havens for registered sex offenders.
Find a good real estate agent who really knows Glendora. An out of area Realtor won't be able to help you find the perfect neighborhood like a Glendoran can.