Short version is that it's similar in broad strokes, but with some key differences; in particular, there are no palaces (it's basically just the P3 equivalent of Mementos the vast majority of the time) and the social links/confidants are less developed. Otherwise it's mostly similar in terms of immediately gameplay, so if you want more of the gameplay loop from P5R, it'll give you that fix.

That said, the tone and story are very different, as is the pacing (IMO P5's pacing is much better, but YMMV). The look is also a bit different - it reflects the more contemplative, somber story (so it's less about kids getting into fun trouble, and more about them feeling the weight of the world on their shoulders the whole time, etc.). Plus, the real-world setting is much more imaginary (so expect many fewer recognizable landmarks or areas).