I've been using MPressionist/MPressionist HD since it came out. I've found it to be a key tool in my MPEG encoding workflow. I verify virtually every encode with MPressionist to be sure the peak bitrate and quality are what I expected. It's especially valuable for determining if the top_field_first flag is set correctly. I haven't found anything else that gives me the same information.

The developer is very responsive to suggestions and help requests.