Edmund McMillen is a professional troll and game designer with a perverted and vile sense of humor. He's also a genius. He's the man behind the modern classics Super Meat Boy and The Binding of Isaac, titles you should already recognize if you've been a Steam gamer for any amount of time.
The Basement Collection is a compilation of his old flash games which made him famous on Newgrounds. Granted, it's an excellent compilation, complete with tons of extra levels and DVD extras, but whether it's worth your time or not really just boils down to the question "How eager are you to play these games again?"
I say this, because this collection mainly exists for historical significance alone. The games themselves are low-res, glitchy, have mildly-to-maddeningly infuriating controls, and most of them are simply exercises in masochism. They’re the type of game you beat just to be able to say you beat it, and that it’s finally over with. In other words, a good challenge. But if you want to unlock all the extras that make this package worth the money, you’ve got to play through the games again. So it all comes down to how appealing that sounds to you.
If these games are important to you and mean something to you, if they were the games that defined your childhood and the games you'd secretly play in computer lab in high school or something, hey, I respect that. Buy it without hesitation and support the man who gave you hours of joy in your youth. We all have our nostalgia bombs and we have every right to indulge.
But if you've never played these games before, there are much better Edmund McMillen games on the market, as well as better games to play for their historical significance alone; I recommend seeking those out instead.
(Oh, and it’s also quite hard to 100%, so achievement hunters beware!)