I've got four units and i've been using them with 2 sb900 and sometimes studio strobes.
They all work flawlessly without any misfire both in manual and TTL mode with the speedlights. They are rock solid and the assist lamp really works (albeit not at close distances like tight headshots).
I prefer to work in manual control mode and i've never been so happy with off-camera flash.
Pros: - Wireless power control for speed lights and standard operation with every flash, power pack or monolight. - Fast and reliable - Rock solid and very good buttons - They are all transceivers and interchangeable - Excellent range - Remote
Cons: - A little learning curve but it takes 5 minutes to understand them, - Exposure settings values are "relative" and not absolute so you have to take a compensation approach (say -2 stops from standard instead of 1/64 of power) even in manual mode..but it's not an issue for me: they always deliver me consistent output on each setting.
You definitely need them.
Yongnuo is no longer a sub-par brand. These triggers are way better than all pocket wizards (except maybe multimax) at an awesome price.