As a fairly experienced Bridge user, I'll add a few observations.
Photo Mechanic provides a fast, easy way to quickly download a large number of images, add IPTC metadata, caption, select the best shots, and send them to a client or editor. It has been designed with the needs of news and sports photographers in mind. It also seems to have been designed for photographers shooting JPEG, although it has been extended to do a good job of handling metadata with raw files.
If you are doing your own end to end raw digital workflow, what Photo Mechanic will offer is more speed in the first few steps. You will probably be working with the JPEG previews of your raw files. While most metadata work is faster and easier in Photo Mechanic, you may find keywording easier in Bridge because of the way Bridge uses keyword groups.
A serious drawback of using Photo mechanic with Bridge is that any ratings you apply to your photos in Photo Mechanic are not carried over to Bridge.
A general problem with Photo Mechanic is the lack of documentation. Development seems to be driven by user requests and new releases come out at a rapid rate. Unless you are an avid reader of this forum and the feature discussions, it is impossible to keep up with the feature changes. IMHO it would be a better service to users to step back from rapid feature tweaking and provide solid user info and a product roadmap, but many users seem to love the rapid response to new feature requests.
