I imagine our efforts at predicting its eventual popularity are pretty much in vain. It relys on many factors, and fun value, technical merit, and originality are only some of them. Whether or not the communities that spring up about it are "noob"-friendly, how easy it is to pick up, how much press it gets, box art, how long people play it, critical reception, whether it is something you'd tell all your friends about, whether wives and girlfriends get interested in it...
In fact, I almost don't care whether Spore is commercialy successful, so long as it developers take notice of it and learn from it. I am actually less excited about Spore the game than I am about the concepts behind it.