As everybody’s said, they’re better than regular jeans, but not much. They don’t have the abrasion resistance of even your basic textile gear. They also generally have no armor, meaning that while you might not get road rash, you still may shatter your knee. But to be honest, my BIGGEST problem with motorcycle jeans is that they just don’t fit right because they’re made by motorcycle gear manufacturers rather than clothing designers. The Shift Torque jeans I started out wearing, for instance, make me look like some combination of trying to hide a pregnancy and hobo. They were too baggy here, too tight there, had a terrible cut, and when I was actually riding (thanks to my sportbike’s very-bent-knee position), climbed up so high on my leg that I was almost showing bare skin even over full-length motorcycle boots. What a rip for $100 jeans.
If you’re going to wear jeans, I’d recommend just wearing your favorite jeans complemented by good leg armor and good boots. When you get to work/school/destination, you can take the armor off (or leave it on). Otherwise, I’d definitely go for an overpant (for convenience) or a more traditional leather/textile pant.