The only two New Super Mario Bros games I played are the DS version and the Wii U version (both Mario and Luigi), and they play differently in my experience. The former felt more like blocky platforming, while the latter felt like the levels have more variation in their platforms to make this interesting.
I get what people said about how the series felt similar, but I think that perhaps if it doesn't play too badly, then it's worthwhile. It helps that the U games have great levels. Therefore, while they are quite competent in how it plays, it doesn't feel like something some people would touch because it looked similar to past games.
I feel that the New Super Mario Bros. games are underrated, but not terribly so (maybe except 2, which stood out the least among the games).
I like all the New Super Mario Bros games as it's the only series (besides Mario Kart) that follows the same formula with every game. Just new power ups and features here and there but otherwise it remains entirely the same.
NSMB2 is quite a lazy rehash, but personally I find NSMBU to be slightly underrated. I'll admit to not having played the whole thing, but I found some of the levels like the swamp level with the painted background, the desert statue levels and the water geyser levels to be quite unique.
For me, not really. 2 was too easy and not enough of its own game, Wii was somewhat innovative for the series (though still not great), and U was an HD version of Wii with new levels and a lame new powerup. I do, however, really enjoy the original - it's very unique, has just the right difficulty, and is one of my favorite games.