I think Ms. Allyson holds her own quite well not only with today's singers, but with the singers of "classic" pop/jazz fare of old such as Rosemary Clooney, Nancy Wilson, Jo Stafford, Lee Wiley, etc. She has her own "voice" and it is one that is instantly recognizable and generally pleasing to my ears.
I've seen her perform a few times. At one of the most memorable ones, I was working as a volunteer stage hand at the San Jose Jazz Festival. We were just getting the stage set for her performance -- I remember I was helping move Colin Bailey's drum set into place -- when the power went out. After waiting for a long time, it became apparent that there the power was not going to be restored any time soon. The venue would not allow any one to come into the theater due to safety concerns, so Ms. Allyson asked Mr. Bailey to take his snare drum out in the lobby and she proceeded to do an "unplugged" set for folks there. She could have easily said, "Screw it. I'm here ready to perform as the contract required, but the venue provided for me is not able to let me perform. Not my fault. Give me my check and I'm out of here". The fact that she still wanted to give some kind of performance, under less than ideal, makeshift conditions, for people who came out to see her is the mark of a true pro. Quite refreshing. I can't imagine, say, Diana Krall doing that.
I was going to mention her version of "The Moon Is a Harsh Mistress". I've heard her sing it live and it was gorgeous.
She's not a bad piano player as well.