Sealing leaks is definitely the best thing about spray foam, but there are some sprayfoam compainies around here that claim the R value rating doesn't apply to foam.....just marketing BS.  R value doesn't have anyting to do with air sealing.  It's just a measure of how fast heat migrates through a given material.

From what I've seen R values are routinely overstated too.  Like my 2" overhead door that claims R17.  It's just got 2" of extruded styrene which will give it R10 at best.  One sprayfoamer was claiming around R10 per inch....really more like R5 or 6.