I can't begin to say it's totally safe. But it seemed safer than I had been led to believe from posts from 1995 and earlier. Riding a motorbike alone late at night is not a good idea. Most motos will be waved through, but cars will likely be searched. Compared to an American city of 800,000, though, Phnom Penh's crime rate is low.
Roads are a mess, though.
You'll see some accidents. Unpaved streets that can become rivers of mud. Can be depressing. Amazing how fast they can move when pouncing on anyone who looks like a foreigner, though. The only overland route is from Saigon, but then you have to get to Vietnam first, which is even more expensive.