Darkspore is a fantastic game. You do not have to play Spore to enjoy this, but if you did, you'll be right at home during customization, which is probably 80% of the game.
The game plays like any other action RPG, move with the mouse click to attack enemies, monitor your health and energy pools, etc. But this game is the BEST of its kind, hands down.
The graphics are phenominal, game play is simple but customization is rediculously deep. Randomized worlds mean no two games will ever play the same. Co-op is great, but not mandatory. You even get daily bonuses for playing each day, like facebook games.
You control a "squad" of 3 creatures in this game, although only one creature is out at a time, you use a simple key press to switch which of your three members you wish to use at any given time. Eventually, you can own many squads, and there's over 100 creatures to find, upgrade, and customize.
You'll select weapons, armors, and utility items, plus the exact placement of these things so you are creating your very own unique creations. You can also spend time giving them a unique paint job if you like, and adding "detail" items to further express your creativity.
There's so many customization options, you will never see or do it all, and for me this is a large part of the appeal. However, if you are not keen on making adjustments to over 100 creatures after every level you complete, customization may take you away from the main game too often.
There are many campaigns, a PvP mode, difficulty settings that cater to the most casual to the hardest core of players. And a unique "chain" bonus for doing several levels in a row without returning to home base to apply the upgrades you found. Chaining many levels together can grant you loot bonuses where you get a much higher chance to get rarer items, but if you fail you lose everything you would have won from the earlier levels. It's a risk versus reward system that pays off well.
I cannot say enough good things about this game. If you have even the remotist interest, you owe it to yourself to check this out. There are free demos available through steam and EA's "origin" service.