It's impossible to evaluate this trade because there are so many question marks, which are really what made the trade come into fruition in the first place.
On Philly's end, they paid a premium price to get James Harden and have to be hoping that he is still JAMES HARDEN. This season has been bad for him. Lazy on defense, out of shape and often injured, disinterested in playing. He appeared to have tanked a lot of his value and in the last view weeks, a bunch of in-the-know NBA people have been ripping him to shreds. This comes on the heels of him tanking his way out of Houston. That's two teams he quit on in basically 365 days. There are a ton of questions about his health moving forward, if he is committed enough to physically get in the kind of shape he needs to be in to continue to play at a high level in his mid-30s, and whether or not he really wants to be a championship-level NBA player.
Philly also gave up Curry, a very good player for them who Embiid reportedly liked playing with, and the kind of lethal sniper that helped Philly's spacing after they lost JJ Redick a couple of years ago. Harden is an infinitely more dynamic player when he is right, but there are also a lot of cons that make it more of a delicate balance. Philly at least kept Tyrese Maxey. If Harden is in better shape and committed to winning and is like 90% of Houston James Harden, it might be a great deal because Harden can be that dynamic second player that you'd want next to Embiid. Given the Simmons quagmire, I think it's probably a worthy gamble. Harden did opt in to his contract, but if things go belly-up you probably could still move him next year since he'd be on an expiring deal.
For Brooklyn, the questions I have are even harder to answer. Simmons is probably the most enigmatic player I can recall as far as how people choose to evaluate him. I'm going to assume Simmons is in great physical shape and excited to play in Brooklyn.
What does Simmons do on offense? He isn't going to be the primary ball-handler, not with Kyrie and Durant on the floor (I know Kyrie can't play at home). So he isn't going to be doing what he did in Philly. In theory he should function as a 5 on offense, similar to Draymond Green. He's fucking your spacing up, so you can't play him with another non-shooter, which shouldn't be that hard for Brooklyn, especially now that that they have Curry and if Joe Harris comes back healthy. But on offense he is going to have to do a lot of screening and posting; two things he didn't seem particularly willing to do in Philly. So he'll have to adjust his game to fit in on offense in Brooklyn and his career history doesn't suggest that is something he'll be really interested in doing.
Defensively he'll be great and help Brooklyn on that end a lot, especially going from Harden-to-Simmons. Brooklyn currently ranks 19th in defensive rating, it will be interesting how much they can jump forward with that swap.
The major questions before Simmons of course are between the ears. He's going to be in a big spotlight in Brooklyn now, especially now that he's filling in for Harden. Will be be able to adjust his game to fit in with a vastly different offense? Will he be afraid to shoot free throws in a big game, which would render him almost useless on offense? Will he quit on the Nets if he doesn't get his way? Could there be a weirder pairing of star players than Kyrie and Simmons? All worthy questions.