I've built up two 650B bikes in the past year; one Kogswell touring road bike designed for 650B, and a Salsa Ala Carte XL size (normally a 26er), running 650B wheels. My verdict: I like 29ers (probably due to my large size and weight)... and I've come to appreciate their ability to roll so well in technical situations. But I also like 26ers, so I believe the 650B may feel like a 29er to smaller riders, and bring some additional rollability to the 26er, without the big leap to a 29er.
That said, my 650B Salsa is one of the nicest rides I've ever had, partially due to the 650B wheels that make it so nice (among other things).
What I DON'T like about 650B is the lack of rim and tire choices... the US market seems understandably hesitant and slow to accept this.
So I've now got bikes that are 26, 27.5 and 29... love em all for one reason or another.
That may not help you decide which is better for you, but hopefully describes 650B as a bonafide option.
Peace,
BB