I know I was really impressed with the first iVista theme by Zach when I tried it so the follow up of a Zen & Today version was a nice find. In general I am more of a Today screen kind of guy.
Zach has teamed up with Neil from BBSmart to create these Zen & Today versions and churned out a good product. My favorite is the Today version which has sharp text and cool effects. The top bar is nice and glossy which makes this theme really stick out.. I think the most impressive part of the theme is the menu. The menu popups are the most refined I have seen so far in any BlackBerry theme. It also uses black text for the menu which fixes the white on blue issue in Google talk that other custom themes have.