I saw the show right before the cast change and was thoroughly UNimpressed with Cassidy - she mumbled the entire time as Kate and her Lucy had the voice down but she showed no emotion, no timing, no subtlety in anything that wasn't a song. Tom is awesome as Nicky/Trekkie, though.
He was very good - Puppetry very good for first week (or so!) so I was impressed, strong singer and actor. Very pleased!
And I don't know how you can say Cassidy isn't good! When I went, she was extremely good as both characters, and put her heart into both - an amazing singer.