I've seen Kate a couple times and I thought she was good, but I just saw Sarah Jane Everman, her understudy, and she was FANTASTIC! Her voice was just so rich and full, much stronger than Kate's. After listening to the soundtrack and Cheno so often - no Glinda has come close to making "Thank Goodness" sound nearly as good - except Sarah. And her comedic timing was phenomenal - she can hold her own with Ana. The only other Glinda I've seen that was funnier was Kendra, but then again Sarah's voice was far superior to Kendra's, so I'll just go out on that limb and say that this is THE GLINDA to see, even if she is the understudy. I should hope that Chicago will get to keep her once Kate's contract is up.
Bottom line: Don't be disappointed if get the Glinda U/S in Chicago - you will get what you paid for - and then some!