Super Mario Bros.: The Lost Levels is a bit of an oddity. "The Lost Levels" isn't even it's real title - This is actually the real version of Super Mario Bros. 2. Japan got this game (As SMB2), but when Nintendo realized (Somehow) that it would be too hard for Europeans and Americans, they changed it into the Super Mario Bros. 2 we know now. Japan later got our SMB2 as "Super Mario USA".
Years later, the Super NES was released, and eventually most of us got Super Mario All-Stars for it, which included a peculiar little game called "Super Mario Bros.: The Lost Levels", even complete with a label saying it wasn't previously released. The game was also later included in Super Mario Bros. DX for Game Boy Colour (Except this time it was retitled "Super Mario Bros. for Super Players"), which is a port of the original game with extras.
So, what is this game all about? As you might have guessed by the two English titles, it is basically just Super Mario Bros. - Same overall gameplay, same enemies, same pickups and everything. The thing that's different is the level design - Every level in the game's been redesigned to be a lot more challenging, with trickier jumps, the inclusion of a few new gameplay elements (Poison mushrooms, blowing wind) and fewer powerups. Basically, it's an even more super Super Mario Bros.!
There's a few small other changes too - The 2-Player alternating mode is gone in favour of "Luigi mode", in which you play as Luigi - He now has differences, namely that he jumps higher and slides further (Meaning it's harder to stop). The graphics have also received a notable overhaul.
There's a few tiny surprises after the final world though - If you beat the entire game up to World 8-4 without ever using a Warp Zone, you get to the secret World 9, which consists of 4 extremely easy (Almost joke-worthy) stages. When you beat the game 8 times in a row (If you did World 9 or not doesn't matter) you go to World A, B, C and D. These contain some of the toughest levels in Mario history and are sure to make your sanity take a turn for the worst.
But is it worth shelling out the money again for what is essentially version 2 of Super Mario Bros.? If you're a fan, yes. The completely different levels, slightly changed visuals and Luigi mode make the game feel like a little more than "just an update". If you're not a big Mario fan, though, I would suggest sticking with the original (Unless you like extremely hard games). Another thing to take into consideration is that this game, in it's original NES/Famicom form, was never released in Europe and America. The All-Stars version had upgraded graphics and sound (And slightly lowered the difficulty), while the DX version cut out Worlds 9 and A-D. This means this is the first time you can own this game in it's original, complete form (Unless you live in Japan or imported a Famicom Disk System and the game!).