I have the thing on all three of my android phones, but use it very sparingly. Why? Because that program is goofy. It doesn't always DO what you tell it to do. I find that a better way to install roms and themes is to download them directly to my PC desktop, then use a USB connection to transfer the zip file to my SD card. then boot into recovery and use the existing CW Recovery to choose the zip I want to install, and go from there. I feel it gives me more control over the process.
In the past, I've had Rom Manager download the *wrong* rom - I tell it to download "Rom A", it downloads "Rom B" - as well as having it decide to install the Gapps file BEFORE it installed one of the Sapphire roms (causing chaos). My advice is to use it with great caution and to familiarize yourself with using CW Recovery to directly install your roms & themes. Why use Rom Manager as a middle-man?
-Mike