Tulsa: The Forever Stems will fit many different cobs. If you go to the Walker Briar Works site
( http://www.walkerbriarworks.com/html/forever_stems.html ), they list the specific cobs that will fit with their stems. You can definitely keep using them after the cob has been retired.
They are high quality, and I have a few, but I also agree with Panhandler that the MM stems are good for clenching, easy to replace, and accept a filter if you're into that sort of thing.
I use both kinds depending on my mood and activity.