Personally, unless you're willing to purchase the upper-level Champagnes, I think the best US sparkling wine producers offer a better quaff. Especially Gruet.

Granted, the limited bottlings from Krug, Ruinart, Moet and Veuve are about as good as it gets, and small producers in Champagne that don't get exported to the States can also be excellent. But the mass-market stuff from the big Champagne houses is not usually up to the standards of Mumm Napa, Gruet, or Roederer Estate IMO. And they cost twice as much.

Each to his own, of course.