TL;DR : An amazing set of original indie flash games, each single one has something great about it. Amazing gameplays, great music, some original stories, a great art style... A must have if you're into simple and quick indie games, and if you like other Edmund McMillen's games. Definitely worth its price.
The Basement Collection is a collection of different indie flash games, created mostly by the creators of Super Meat Boy / The Binding of Isaac.Story / Setting
Most of what the games have to offer is their gameplay and art style, but some of these games have a really unique story. Meat Boy and Spewer barely even have one for example, whereas Coil is really story driven, even if its story is not really clear, and it requieres your own interpretation of the game. Time ♥♥♥♥ and Aether's stories are present and interesting, but really aren't the core of the games. Finaly, if you unlock AVGM, I'd say that its the game that has the most meaning in the list, this game is litteraly a parody of other games and is only worth playing if you understand what the games is making fun of (or if you want the achievements !) Gameplay
Trust me, there's going to be at least one game that you'll enjoy playing in there. Most of the games are hard to beat completely, like, really hard. Even if you reach the "end" of the game, there's going to be advanced levels or achievements that will take a while to beat and obtain.
I guess you already know how Meat Boy works, since it's basically the original version of Super Meat Boy (one of the best and most challenging platformer I know of). Spewer is an amazing platformer as well, but requieres a bit more tactics than Meat Boy. Aether is like... the smallest sandbox game I've played, it's fun to play and really is different from all these really hard games. I didn't play Triachnid much but it's gameplay is quite unique, even if I found it a bit boring after a while. Grey-Matter is like one of these old Starship arcade games, it's hard to describe. It was fun but I'm a noob at it :( Time ♥♥♥♥ was my favorite one to play and has an awesome idea behind it. It requieres a lot of attention and reactivity, especially in the harder levels. I'd say that it's half puzzle, half platformer, it reminded me of Fez in a way. The only problem with it was the saving system, make sure you click "exit" on the right, under the person who speaks to you (no spoiler) so that the game saves your progress, and try to quit the game with the in game option (top left). Performance
Knowing it's a flash game... the performance was pretty bad for what the games had to offer, but it was totally playable. I did have to lower my screen resolution to play Spewer and Aether properly, as the games were quite laggy (kinda like the first Binding of Isaac was, for me). The game can't be rendered by my GPU (because it's flash I guess ?!) and uses integrated intel graphics :( Probably won't be an issue if you have a desktop PC with more a more recent CPU. Style / Graphics
As always with Ed, i'm not dissapointed. It's not as full of weird wonderful things as The binding of Isaac, but pretty much each game has it's own look and feel. Triachnid and Aether were especially enjoyable to play simply because of their art style. Time ♥♥♥♥ wasn't especially nice visualy, (I know that the pixelated look was what was desired) but the game clearly isn't about it's graphics. Finally, the music. Wow. Danny Baranowsky is by far my favorite video game soundtrack composer. If after playing Time ♥♥♥♥, Meat Boy and AVGM, you don't have the melodies stuck in your head you must be deaf.