To answer your question, no it does not tie up my PC while watching the HBOGO channel via PlayOn. It is a process running in the background but I have never had any issues in that respect. It works quite well overall other than requiring you to log into your Comcast account every other day or so on the PC - but I have that issue anyway with other Comcast apps.
I dislike Comcast as much as anyone else on this site but this really is an inexpensive work-around to watching HBOGO on a Roku if you have a PC accessible anywhere on your home network.