A 650b with a 38mm tire is the same circumference as a 700x23mm. A 650b will technically accelerate faster than a 700c but will technically not hold its momentum once rolling on the flats as well. A 650b wheel build vs a 700c build with the same components will be stronger and lighter, although not much but there is a difference. You can find lots of tires built in 650b, but finding one on the road if you flat may be difficult, so having a spare is a good idea. If you have cantilever brakes and the posts are set for 700c your screwed, you have to look for a custom frame in 650b sizing, or you need to find long reach caliper brakes, or if disc brakes then your good.

I think many ppl, including Jan Heine publisher of BQ will tell you they are faster and much more comfortable than a 700c. It will most likely change your gearing ratios, but that is easily fixed.

The main benefits seem to be that you can have big cushy comfortable tires while using a fairly standard frame size and it keeps the wheel circumference within reason.