I grew up in Haymarket, which is just outside Manassas. I loved Haymarket - I grew up in the country, on Bull Run Mountain, and while it wasn't the most social place, it was an amazing place to play. I spent half my childhood out reading in the woods, and it was so beautiful. It's getting awfully built up, though we have family jokes that Civilization has come to Haymarket, because there's are now two grocery stores and you can get pizza delivered from two different places. The local cops are terrible, and if you speed at all in Haymarket, may God have mercy on your pocketbook. But it's a beautiful area, and I've never regretted growing up there for an instant.
Manassas has it's good and bad areas, like anywhere else. Most of my closest friends live there now, and have stayed out of any mischief. The main area to avoid is Coverstone (known locally as the Coverstone ghetto - it's not that bad really, but there's a lot of meth, a lot of theft, and a lot of noise), and the most built up parts of Old Town (last I heard, the prostitutes hung out in the Sheetz. They may have been thrown out by now). I can ask my Dad if you'd like - he's retired military and an active real estate investor. He's lost two houses in Manassas, one when the gangs moved into the neighborhood and dropped the housing values like a rock, and the other when he rented it out to nightmare tenants who destroyed the entire thing. But the house in Haymarket has stayed fairly steady, except his is on a big plot of land that developers want, so they've been trying to tax them out for years.
Also, if you haven't looked into it, Reston is an absolutely gorgeous area.