I'd say with an average razor and average skill it takes about twice as long as shaving with a safety razor. My razor isn't the sharpest out there and taking off a mustache makes me feel like crying. So long as you never pull parallel to the blade, it's pretty impossible to cut yourself. I've sliced myself numerous times with safety razors but never once with a straight razor (I have nicked the tops of pores and zits, but never really cut myself). If you don't shave every morning, then a straight razor is way easier because it doesn't clog up.
It can be tricky in certain areas because they are really designed to be used ON you not BY you. Back in the day you'd have a barber or your wife shave your face. The double-edged razor I linked a few posts back is pretty ideal. The closeness of a straight razor without the high cost of a cartridge razor or the awkwardness of a straight razor. I have used all three kinds of razor and Double-edged are definitely the superior choice.
I just wish I could find something that doesn't irritate the shit out of my face. I've tried everything from 2 to 5 blades, and none of them do it. Mostly what I'd shave is my neck anyway, since I have a beard that I've grown attached to.