The first question to ask would be where you stand on the NSMB series in general. Thematically it is exactly what you'd expect, so if you're hoping for a fresh aesthetic or visual novelty it'll probably disappoint you. It was also heavily promoted for the coin collecting elements, and although coin rush is a fun mode to noodle around with for a while if you're into speed running or score attacks, the larger meta game of collection a million coins could come across as inconsequential at first blush.
That said, it's my personal favorite of the NSMB games, and imo by far the most underrated. Why? Because in spite of its samey look it has the best controls, level design, and powerups of the series. I feel like somewhere along the way when people caught wind of the fact that the main development team wasn't working on this one (they were busy with NSMBU, which was being made simultaneously), a narrative formed about how NSMB2 was a second tier "B-team" effort. It was also easy to dismiss as a NSMB game wedged between NSMB Wii (a fulfillment of what the original should have been) and NSMBU (the first side scrolling Mario in HD, with more vivid backgrounds). I think because NSMB2 did little to differentiate itself visually it fell victim to franchise fatigue, and with a WiiU version on the horizon it was regarded as a sort of stop gap.
What I personally found though is that the layouts of the levels are more thoughtfully layered than rest of the NSMB series, and more akin to SMB3 with multiple viable paths through many of the levels and a greater sense of exploration and discovery without ever meandering or becoming unfocused as a platformer. It also has more of a tendency to switch more freely between horizontal or vertical orientation both overall and within the space of individual levels, including scrolling right to left at times, which makes the layouts feel more organic and varied in their approach. And although going coin crazy doesn't have a ultimate payoff as far as a meta reward, approaching the levels in that way helps to underscore the attention to nuance that permeates the levels from a pure design standpoint. Just completing levels is fun, but pushing them to their limits yields a beefier experience that might be easy to overlook or take for granted otherwise, and it's much more consistent and integrated than the standard 3 star coins alone could ever be.
It also shares a good amount of SMB3's powerups, which for what they lack in originality remain the most fun and well rounded of the sidescrolling Mario games imo. The difficulty curve and pace is more satisfying than other mainline NSMB games as well, Not that it ever gets insanely difficult, but it has a steadier progression rather than plateauing around the third world and never really picking up beyond that, which is an issue I have with most modern Mario games (which usually overcompensate by tacking on a final "bonus" level that suddenly requires all the skills the main quest never bothered to gradually acclimate players to using by the latter worlds).
The controls also strike a nice balance, remaining tight and responsive, but also giving Mario enough weight and momentum to feel tactile, which in turn makes just playing and navigating obstacles more pleasing.
At any rate, you've got plenty of people telling you not to chance it. Just wanted to offer a counterpoint with some specifics on what's good about it. Basically, if you're a fan of platforming games and you can get over rehashed level themes you should really give it a spin.
