It's all about the feeling.
Spore, as a game, is something of a disappointment. The cell stage is a fl0w-like minigame, the creature stage is awfully simplistic, the tribal and civilization stages are extremely dumbed-down RTS's. The space stage, however, is quite satisfying, although not without it's problems. If you play as a warlike race, you will be attacked every 5 minutes, and so you trying to make any progress quickly becomes a chore. Play as a friendly race, and the missions will get repetitive.
However, the gameplay flaws can be easily overlooked. This may sound idiotic, since this is a video game, but the main appeal in Spore is not the gameplay - even if the gameplay was flawless and properly fleshed out. The editors are what make this game special. From a cell, to a creature, to cars, to spacecraft, to buildings. Absolutely everything can be created (except plants). You have absolutely no limits. Your creature may be a giant 5-mouthed monstrosity or a wide-eyed baby elephant. Think you can be cruel and give your creature no hands? It will grab things with it's mouth. It will even dance without legs. Later on, you get to give it clothes, at first for a slight gameplay advantage, in the civilization stage it's purely cosmetic. Creating cars and spaceships are also a breeze, just as deep and wacky, as are the different buildings you can create.
The editors are by far the best reason to own this game. If you are even slightly creative, you will lose hours of your life creating buildings, creatures, cars, spaceships. And these not only benefit you. You can upload all of them to the Sporepedia, and other players will have them in their games. Just like you have other people's creations in your games. You will never encounter the same species twice while playing, which is pretty cool.
Having said all of that, there's one more unique thing about Spore, and this only applies once. When you first get it, clear your program for the day and night. Install it, and start playing. Start out as that tiny cell. Eat, grow, run away from that enormous spiky mouth, eat some more, grow and eat that thing with the spiky mouth. Eat, grow, run away some more, climb out of the water. Get your legs, meet other creatures, interact with them. Destroy some of them, befriend the others. Form your tribe and conquer the others. Watch as your species takes over the globe, and your faction wins out. Go to space... and realise there's hundreds of other planets where this exact same process is still happening. Go to another planet and watch as a tribe is attacking another tribe's village. Watch random creatures sing to each other from the comfort of your spaceship. And realise, just 8 hours ago... you were that creature. You were that tribe. You started out as a tiny cell, running away from the larger predators - and now you're the ruler of your solar system and soon of the galaxy. That feeling of epicness is one of the most precious moments you will experience in gaming, and for that, I fully recommend for you to go and buy Spore.