Interesting signing. For £35-36m I think the chances of him not ending up as a "good signing" are very slim...
...but we need to be patient with him. He doesn't have great numbers for a striker, and I'm not expecting that to change in his first season at Old Trafford, but he brings a lot of tactical flexibility upfront, and can do a lot for the team and play well even if he doesn't score or assist, and often makes his teammates' job easier with the things he does. I also feel like he's someone who can develop into something great, but that should take years and he's definitely not there yet.
I'm also worried that he won't take it well if he just becomes a bench player and Hojlund's sub. I mentioned it in this thread earlier today that according to people who know more about him and watched him extensively, his mental profile suggests that he's not ready to just be an impact sub at this stage of his career, and even before we were seriously linked with him, I thought this transfer doesn't make a lot of sense from Zirkzee's point of view, as making the first big move of your career at 23 should be to a team where you're the starting #9. With Hojlund already in the squad (who's definitely a better player and a bigger prospect in my book), and us looking at strikers in addition to Zirkzee according to the Athletic, I think this could cause issues. I think the best decision from a squad harmony perspective would be to start the season with Zirkzee as our starting striker ahead of Hojlund, unless we sign another attacker (depends who, I don't think Toney could be benched for Zirkzee, for example), and then if he gets benched for Hojlund, I think he will take it better than if it was already the case from the beginning. He should easily get a lot of game time, though, be that as a starter or from the bench, so the overall amount of minutes shouldn't be an issue.
Overall, I like the move, he's a floor-raiser signing that gives us a lot of flexibility and variety upfront, and could also massively improve into a top player in a few years' time. For the price reported, this seems like a low-risk, high-reward investment.