I really like the Play Money Sit and Gos on
PokerStars. This is a good place (in my opinion) to learn a lot about the game. The play money tourneys there go pretty much like the low buy in real money tourneys (once you get past the one All In Nut). People take them pretty seriously and you can learn a lot.
I wouldn't say the same about the Ring games though.
The other thing that is interesting that I have noticed is that the Play Money chips have value. Many were sold on ebay and now that ebay has banned their sale, they are selling on Yahoo Auctions. (auctions.yahoo.com) People also come into the play tourneys and solicit others to sell them their play chips.
Why would people buy them? I don't really know, but they do have value. I haven't seen any other play chips selling for money anywhere else.