If you love Legend of Zelda a Link to the Past ... then you'll love this. Or if you have kids, it's great!
1) It's not an MMORPG. Playing WoW was great but then I realized I didn't have enough time to put into it. Having to do side quests and level up constantly with long campaigns and dungeons didn't appeal to me anymore because I work. With Spiral Knights (whether you pay or play for free) you can run through a Tier in a matter of an hour. Then it's up to you if you want to continue or not. You can spend all the time in the world as you please or you can log in and play for a little and enjoy the levels.
2) It's not complicated: If you have kids who are too young to play WoW or any MMORPG this is fun and easy. Yes they will need help learning about recipes and making gear but the list is very small and not as expansive as typical MMO's. Also the graphics are kid friendly yet enjoyable for adults. It's not a heavy burden on a system so anyone can play. I love it because a Link to the Past was my favorite game growing up and this is a nice nostalgic game.
3) Pay or Not to Pay, that is the question: Personally I don't like this because I'd rather spend $40 or $50 on a game and then get to play it forever with no road blocks, but on the flip side Spiral Knights does a nice job allowing people to play this for free. I've seen people beat the game without ever spending a single dime so that was nice to see. But if you can afford it, why not thank the game dev's and pay a little. Also, not monthly fees, thank god!
4) The Future: I'm excited to see how this game evolves. It's nice to see a small independent team create such a great game and I hope to see them add to it monthly. keep it coming.
5) Just plain Fun!: Going down levels with three friends helping each other get more gear and beat the end bosses is great. Just try it.