I find the carbon 1/2 play money tables to be decent. Not GREAT mind you but they take it seriously enough you can work on your own game risk-free practicing counting outs, pot odds
, learning the software, using any 3rd party stuff, etc as well as just general playing practice. I've run into several players who play as if it is cash (ie: I've never seen them showdown rags hoping for luck etc) I usually tag these players so I know at a glance I can take them seriously.
I suppose if you go significantly high in stake levels even play money types will play somewhat seriously since it took them actual work/time to get to those levels.
Agreed though eventually you will hit a definite wall and will need to go cash if you want to continue to improve. Biggest trick when learning using play is to play at your best. If you get a suck out, use your hand replayer (if available) to look it over and make sure you did what YOU were supposed to. If you played your hand properly, then don't worry about the donks winning their fake money. They are just getting better at being bad so if they ever go cash it'll add some extra to the poker economy for the rest of us