I've been using PM for 6 years now. I probably spend more time with it than any other photo related application.
PM is NOT a photo editor. It is a image browser and organizer. It is NOT a catalog program. It simply uses the folder structure of your OS to store photos. That has advantages (its simple and does not rely on always using a catalog program to keep track of you pictures), and disadvantages (it is slower to search for a particular image based on keywords and such).
PM is great at ingesting photos (copying the images from your card to your hard disk or other location). While it is doing that, you can remane the images, add descriptive information in XMP/IPTC formats, and also have them copied to a second location for backup.
Once on your computer, allows you to select images (tag) rate them (0-5 stars) and assign color classes (nor or any of 8). This allow you to essentially catagorize teh image for viewing and processing.
Yo can also extract the preview jpg in a NEF file into a separat JPG, as well as do some "soft cropping) that you can apply when saving the copying the image for anotehr use (such as uploading to Nikonians. You can also target the size of the saved image so it fits within the Nikonians forum 300kb limit. at the same time you can save the new image to web-friendly sRGB color space (if you shoot in adobeRGB) and sharpen it.
Photo Mechanic is also a hub for launching you image into a photo editing app. I primarily use Capture NX2 and have that as my default editor, but you can also set up any other program as default, or have different default applications based on the file extension (CNX for NEFs, Photoshop for PSD, etc.) You also have the choice of using any other as an optional "Open with" program. I find this useful for programs such as Photoshop, NIK SilverEfex Pro, Noiseware, Perfect Resize, ExifTool, Photomatix, and 9 or 10 others.
If you think you need a cataloging program, Camera Bits is working on an add-on module for PM, but it is behind schedule.
I can't recommend PM highly enough. It is a great program, very powerful and very flexible. Plus the tech support is excellent.