It's pretty decent. The fundamental gameplay is solid and there are a number of much-needed quality of life changes (e.g. optional tutorials, fast-forwarding through cutscenes). Where the game slightly falls flat is it kind of feels like it lacks character, especially after OC-heavy games like Superstar Saga and Dream Team. The story is minimal, the enemies and bosses are all Mario enemies you've seen before, the locations are very standard and if you were hoping for more of a substantial crossover with Paper Mario, then you'll probably be disappointed: you get paper versions of the main cast and various enemies but not any of the series original characters.
I'd still ultimately recommend it, but it depends on what things you value in a Mario and Luigi game: if you're looking for that good old fashioned M&L gameplay, you'll be fine. If you're looking for bucketloads of charm and originality, best look elsewhere.