You remember the first evolution stage in Spore? Someone decided to make a full game out of that and it surprisingly ended up being pretty good.
You start off as a little cell in a large map and grow by absorbing other cells, but to do that you will have to move, which costs a bit of your own cell, depending on your current size. Continue to grow until the game says "all right, that's enough, you can move on to the next one now.". And that's about it. There's a couple of little gimmicks here and there and different gravity every now and then, but that's about it. Critics often say that Osmos is a very relaxing and immersive game. Immersive, yes. Relaxing? Hell no. Once you get to the harder stages, the game starts getting pretty frustrating and relies on some luck under certain circumstances. During some of the last stages, I was moving myself very carefully, hoping to god that I wouldn't get killed by one of the larger cells that was barely a pixel away from me. So yeah, claiming the game's relaxing is a lie to the biggest degree.
As for the presentation, I like it. All the cells look great and filled with little details when looked at from up close, but as for the backgrounds, they're basically non-existant. Not that I have a problem with it, considering you're going to focus on the cells for 99,98% of the game.
If there's one thing that Osmos is known for, though, it's without a doubt the music. The OST's been praised for being, wel... good. And in this case, I can't deny the soundtrack's pretty damn good. (even though it really doesn't fit in with the player desperately hoping things don't go horribly wrong every stage)
Overall, Osmos is pretty fun, but for 10 bucks, I can't recommend it at that price, but if you can find it on a sale, I'd definately recommend picking it up then.