I've used the Varizoom zoom controls. For the money, there fine. I find them to be bulky and the construction seems a little cheap. The zoom rocker also feels a little ackward to me. For $500 bucks, it's fine.
I've been using the Vocas zoom controls lately. http://www.vocas.com/ They're a lot more expensive. The rocker is very responsive, allowing for extremely subtle zooms. The construction is sturdy and feels good in your hand. There's a digital read out that helps you reset the zoom position between takes. Two of the models allow you to control both zoom and focus or iris, if you have a compatible lens.
I also like the Preston Cinema Systems - Microforce. http://www.prestoncinema.com/products_micro.html This is a film industry standard that also works with video lenses. You control the zoom with a pressure sensitve button rather than a rocker. It takes a little getting use to because the button barely moves. Despite that, it's very responsive. (you have to try it to see what I mean)