I haven't had experience with column grid arrays. I would think you'd still have the concern that solder doesn't flex as reliably as copper or aluminum. Aren't there issues with maintaining column stiffness under stress? :-)
@betajet: My chief worry is stresses on the balls caused by thermal mis-match between a BGA package and its underlying PC board.
Being a string man, I'm going to totally ignore the opertunity to make jokes about stresses on your balls due to thermal management and instead ask if you've had any experiance with column grid arrays (CGAs) in which the solder balls are replaces with solder columns that mitigate against thermal stress (much like wearing looser undergarments :-)
FYI: Disadvantages of BGAs. My chief worry is stresses on the balls caused by thermal mis-match between a BGA package and its underlying PC board. Those tiny solder balls don't flex like proper metal leads -- they crack. And who knows whether those mechanical stresses are causing tin whiskers, since that phenomenon is not well understood. "Oh my ears and whiskers!"