Keywording and tagging are never fun. It's repetitive detail work that I doubt anyone really enjoys very much.
I primarily use Lightroom2 (actually 2.6RC now, and have toyed with LR3 beta a bit).
With Lightroom you can add a lot of generally descriptive things when importing the images. You can even automate a lot of this if you wish. Then go in and add more on a per image basis as you edit... Or in small or large batches as you see fit. It's very easy, but I haven't used Photo Mechanic so can't really compare.
CS3/Bridge was just way too slow whenever there was a high volume of stuff to be done. I used to use it but have pretty much switched to LR2 for all the preliminary editing, up through the proofing out to customers. This has cut my editing time by about 2/3 (what took three full days now takes one).
After the customer has made their choices, depending upon usage, the export from Lightroom might be usable... Or if necessary the final, selected images are re-processed individually through CS4 or Canon DPP.
Keywording can be added at any point along the way, but I find LR to be the easiest and quickest place to do it.