I quit using push in about 10 years ago when I realized it was causing issues when I had a few heaters plugged in at job sites. My detective skills told me that 10 fried outlets over one winter and all 10 with the push-ins in use might be a clue to an issue.
So when can you use it? And even more important why do manufacturers still produce them with that option (and if you want to take it a step further why does big box still sell them?) ?
See question Why do electricians prefer not to use the push-in connections on receptacles? for more background.