I'm barely starting my first playthrough of FE5 (just got to Ch. 7) so I can't speak of it as a whole, but as far as first impressions go I like it more than dislike so far. My biggest complaint is how unnecessarily cryptic it seems to be. It might just be due to an unfinished translation, but a brief "I won't leave anyone behind" being used as a way to explain other units being captured if Leif escapes first is hardly an explanation. The fact that you can miss both Lifis and Asvel (two characters that I can tell are almost crucial to these Manster chapters) is awful design. Also dodge tanking seems to be a big focus in some parts. All that said, I dig the map design and composition overall. I love when FE makes you use the most of what you've got. I'm not a fan of juggernauting so having every unit have a role to fulfill is awesome. It's part of the reason I like the concept of a fatigue system since further seems to prevent excessive focus on one unit (at least early game it does. I have a feeling that as HP gets higher and the roster expands, it'll end up being less of a concern).
I'll likely have a lot more to say once I've finished the game, but so far I love how much it not only did different but also clearly influenced a number of elements in future games.