I have a player who has used it and it's really not as great at it seems at first glance.
The simulacrum can't recover resources, and has half hit points. Over the course of an adventure, it is going to have a relatively marginal impact. Let's say that the adventure is a week long. The wizard hasn't doubled his spells. He's increased his capacity by 1/7th. And that's if the simulacrum actually survives, which is hardly a given with its HP.
The biggest impact is double spell casting in those rounds that it is used. Which is potent, but given how HP scale relative damage spells, by no means OP IME. If it were, dipping 2 levels of fighter (for action surge) would be a no brainier for a wizard.
Maybe at level 17+, when you can wish a new one into existence for no cost, it might be a little strong. But they still have to expend their 9th level spell for that day, which is a big opportunity cost. If you do it, you don't have Foresight or True Poly or any other 9th level spell for that day. Additionally, most things get a little OP at 17+, so it isn't something I stress about.