This is a sweet looking frame. I like the idea of fat cushy tires on a road bike like this a lot. One possible downside to 650b wheels/tires, however, is cost. The rims I've seen by and large tend to be pricey. This is the best price I've seen on a very decent looking built up wheel. Suzue 650b Trekking and Commuting Wheelset
Another possible downside is clearance for a sufficiently fat tire so you don't drop the BB too much; heel strike is no fun. Here are some pretty decent guidelines in thinking about a 650b conversion, 650B Conversion Guidlines