Standx's mainnet for 20 days, and data is currently ramping up slowly. You can now trade using your Binance wallet. Importing your own wallet for trading also offers bonuses; you don't necessarily need a Binance managed wallet.
It seems that the partnership with Binance is quite deep, after all, the founder was previously a senior executive at Binance Futures.
Some people complained about the limited number of trading pairs and the UI not being to their liking, but I don't think it's a big problem.
It's incredibly smooth and fast, which shows that the underlying architecture is very well done. Those who know, know.