I've had quite a few people ask me about the HOHO (Hop On Hop Off) buses and there were 5 different companies that I saw when we were in Rome in January 2013. Below are the flyers that I picked up for each one except the Roma Cristiana Bus. For some reason they would not let me have a flyer (at least the two times I tried). I tried at a place that was selling tickets for all of them, and I tried at one of their buses. I could look at it, but I couldn't keep it unless I bought a ticket. Here is the website though for their bus: Roma Cristiana.
If you need a clearer picture of something, just let me know. And I did not take any of the buses so I can't give my opinion.
Since I added this page I have read that there are 7 HOHO buses in Rome. Rome Toolkit will give you an overview of all 7. I suggest that you go there and check them out.
First up is the Rome Open Tour.
Next up is the Green Line Tours.
Ciao Roma Open Tour
And lastly City Sightseeing Roma
I hope that helps!