[Using Brooklyn as an example.]
As a way of complimenting a show without being deemed a shill, I would avoid the following types of words and/or phrases:
brilliant
fantastic
the best show I have ever seen
fabulous
genius
and virtually any superlative.
[Feel free to add any give-away adjectives of your own.]
Comments like that will lead to a letdown and bad word-of-mouth and could very well hurt the show you like. You might want to comment that it was a show you liked a lot and is worth watching for, and leaving it at that.
Tell us WHY you liked the show not THAT you liked it. Let us know via your comments that you've got some brains and discernment. Through your comments you will be able to tell us whether you've seen it or not. I get the feeling that the entire population of Colorado was crammed into a small theatre in Denver to see this untried and unsung musical.
Don't bleat about how brilliant-fabulous-great the show is especially when it is still in rehearsal. You haven't seen the show that is coming in. Once previews start things change.
And why pay full price for this show when there are discounts floating around.
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher