The Yongnuo's are pretty good. Generally they're built well, except in places (battery cover I'm looking at you) their cheap chinese manufacturing shines through. Be careful and no ill shall befall thou.
If you plan on doing any action photography with them, here's a link to the measured duration:
http://www.gock.net/2012/01/flash-durations-small-strobes/
Notice how the Yongnuo outperforms the SB-28 at all power levels.
Quick and dirty example:
Imagine you're photographing a skateboarder using flashes at 1/4 power, duration = 1/2809 s:
Average skater travels at 15mph (googled, it's a rough number), or 6.7 m/s.
6.7*1/2809=2.3mm motion during the flash. Take into account that the flash intensity is not constant during that period, so even less motion is noticeable.
Basically, these cheap little things are fucking rad.