Happened across this at Sportsmans Guide, figured somebody here might be interested in these:

http://www.sportsmansguide.com/net/cb/cb.aspx?a=499712&pn=2

I'm not personally likely to buy one, as I don't have any room left in my glove compartment, but I am curious if these suits are still good. Do they have an expiration date? I assume they must have been rotated through.

They look OK. They are certainly much cheaper than the comparable Mustang immersion suits.
Yes immersion suits work.
The idea is that if you are in a severe situation you put the suit on, and continue working.
Then if you are capsized, washed overboard or sunk you are protected from freezing long enough to get back on board or into your life raft.
The thing is that being dropped into freezing salt water unprotected usually means less than 5 minutes survival time, with the suit on the crew might have enough time to retrieve you or you might be able to effect a self rescue.

Neoprene is what they are usually made of and that is pretty much decay proof, but they are surplus gear and that means you inspect first them if you can.
I have seen the kind of damage some quartermasters do in a misguided attempt to deny civilian use of military gear.
I saw a whole truck load of obsolete wool overcoats with the backs slashed and one arm cut off for just that reason.
Still it looks good for the price, even if you had to patch a couple of slits in them.
If I was still on the waters instead of the prairies I would likely order one or two.