I haven't bought any because I want to be a domain seller, and not a domain collector. I prefer to focus on .com/.net/.org and ccTLDs. It's already hard enough to make sales in good TLDs.
I simply don't think the risk/reward ratio in new extensions is good.
Historically, all new extensions have been failures. Why do people think that releasing hundreds and thousands more would produce a different outcome...
It's not that we shouldn't be looking to the future, we should. But let's not discard the present, and what already works today.
Anyway, they are not mainstream and I doubt they will ever be. But some people said that after 3 or 5 years they would be widespread (sounds familiar isn't it).
Quite honestly, I don't like them very much and at this point I would never use one for a real life business. I know that many TLD peddlers wouldn't either, so they don't even believe in the virtues of their merchandise but they still expect that somebody else will see value where they don't.
I could write a long post, but I will now cut to the chase:
A good name is a name that is already valuable today. In general, expecting that something not valuable today will become valuable in the future is wishful thinking, and it doesn't work. If you are not making sales after a while, not even getting inquiries, you must question your strategy.