While it's not flawless in my mind, Super Mario Bros is an undisputed NES classic
Considering that Super Mario Bros is over 25 years old, it's remarkable how well it has aged in most regards. The gameplay still feels tight (if a bit on the simplistic side), the graphics are sleek, and the level design is still competitive with today's platforming titles. This is a game that virtually all age groups can have fun with, and while it does obviously show its age in a few inevitable regards, Super Mario Bros is just as fun today as it was back in 1985.
With that said, I don't think the game was ever entirely flawless - the game is quite challenging at times, and a few of the later levels which infinitely repeat unless you jump in a certain pattern are quite frustrating due to the timer. If you're not playing this on an emulator with save slots, this is a pretty tough game, and unfortunately, I don't think the game is great enough to warrant a lot of the frustration. The level design is relatively repetitive, and the 'run-jump' mechanic just doesn't feel like a whole lot of fun to me. Granted, platforming games aren't usually my preferred poison, but I do feel like the jumping mechanic really tests your patience a bit too much. This was a moderately enjoyable experience for me, but not the jaw-dropping, life-changing event that most gamers tend to agree on. Who knows, maybe I needed to be there in 1985 to really appreciate this game.
Either way, Super Mario Bros is a pretty short game, and it is something that everybody should play since it's so historically significant. This is a classic NES title, and although my own personal enjoyment is rather limited, this is a good game that I did have fun playing. I doubt I'll ever play it again, but there's little doubt that this is one of the best 8-bit platformers.
Don't take my word for it, though - definitely play this game and form your own opinion, especially if you like platformers.