I find the Trulia crime map misleading. Even reading the notes won't give you the full picture if you're not already familiar with the area.
Busy and heavily populated areas often show up as bright orange, even if they're generally very safe. More people and more businesses = more chances for incidents. In Chicago, most of the Loop, River North and Old Town look like a criminal's playground on Trulia, even though they're some of the most expensive and safe areas in the city. The same goes for parts of Wicker Park and Lakeview.
One thing you can generally count on, though: if you see a lot of crime in a solidly residential area away from a major intersection, it's probably not great.
Heat maps are an awesome way to quickly visualize data, but only if that visualization is done meaningfully. It's not very meaningful when credit card fraud and shootings are treated the same. I would love it if there was a way to filter Tulia's map by different types of crime. Spotcrime.com does a better job of this this with their icons, but they don't heat map their data.
Trulia, if you're reading this...add some filters and you'll have a lot of grateful RE investors!