Voted a C:

Pros:
Excellent range of ways to use points for hotels pre-devaluation.
A vast array of hotels across many brands in many places to earn and burn.
Strong in extended stay/budget hotels (HS, HGI, Hamptons)
Free breakfast at most properties/brands for Diamonds.
Rarely have had a problem redeeming points.
Easier to get great rates than competitors (but that probably results from many of the cons I mention below)

Cons:
Devaluing points in the midst of the worst recession since 1929 recesssion makes no sense at all.
New points chart is awful--with little relief for Diamonds.
Hotels are inconsistent (physical structure and service), especially in luxury and upscale--WA, Conrad, FS Hiltons).
Diamond desk is worst of any major chain--incompetence and lies are standard fare.
Far too many properties do not treat Diamonds in any special way.
No lifetime elite status (which is why for several years any nights above 60 have gone to Marriott).

Overall would say B- pre-devaluation, D- post-devaluation.