New Super Mario Bros, I think can be described in just one word. Good.
It's not fantastic, I know I've played better platformers, more memorable games with a lasting impression and from around this time. And yet, I will always compare these games to the quality of Good that Nintendo and this game has set here.
Back in the day this was a big game, as it was the first new 2D Mario game since Super Mario World 2: Yoshi's Island. Not that gamers were lacking with 2D Mario with the onslaught or re-releases on the GBA. But looking back on it now, this game is lacking quite a lot in terms of power ups, there's only 2, and the very linear level progression.
There is a lot more replay value than I remember, especially with worlds 3 and 7 being optional. But at a time where I can play New Super Mario Bros Wii, or U on my Laptop, there's very little in the way that makes this game stand out today.
It's gimmick is short lived, it does nothing which previous Mario games didn't do already, or did better. And while it is a very fun game to replay, my entire time replaying it, made me want to play another game.
Which brings me back to the top of the page. If I had to describe New Super Mario Bros in a word. I would use good. But I know it could've been better.
I remember at the time that this released, just having a decent 2d platformer on the DS felt like a novelty. No, it doesn't do anything special or super interesting, but as someone that wasn't exposed to the older NES/SNES games really, this was a good introduction to the series at the time (which was admittedly completely overshadowed by Mario 64 DS around the same time).
Out of all the NSMB games, I think the original Wii release is probably the best one IMO.
I had the Mario Advance games, plus Mario Deluxe on GBC. For me it was a natural step with a modern feel. Personally I prefer Mario U, mostly for the rare levels with the unique aesthetics.