When I was just there just two weekends ago, Mission:Space easily had 40-50 minute wait for most of the day while Test Track had 60-70 minute wait. MS is a people-eating machine, which really helps with the wait times. In comparison, Test Track, coupled with its freqeunt down times, does not load people nearly as fast as MS. So to compare wait times does not reflect the popularity of the ride.
No offense, but I completely disagree with Brer Fox. It is his opinion that he does not like the ride, but I disagree that it is not popular among the masses. MS is my favorite ride at any park and is likewise the favorite of many of my friends. Everytime I ride (which is a lot), there is always that excitement or buzz from other riders that can be heard, indicating it entertains a lot of other people too. I have ridden MS countless times, usually several time in one trip to Epcot if possible, and love it more everytime. You can't get that experience anywhere else in the world.
MS's re-rideability is no different than any other ride in the world. With the exception of TOT and to a lesser extent Indiana Jones at DL, every ride you ride through does exactly the same thing everytime. Also, MS is interactive to an extent. Sure, if you sit back and do nothing, the "computer" will take over...but you do have some control. Try hitting your assigment buttons a little sooner than when CapCom says NOW, it will trigger the effect before its ready and your pod does little things like tremble a little longer and make other sound effects, etc. It is not huge, but once you've been on it a lot and play around with things, you do realize that its more interactive than it let's on. Not to mention audio cues can change if you don't press your buttons (Computer Over-ride audio, for instance). Also, I just love pushing all the buttons and levers that do nothing than to switch on and off lights, especially during the high G moments. The joysticks do nothing other than enhance the believability that you are steering the ship....use your imagination a little. The joysticks also play another role...the shaking of it goes through your arms thereby simulating a rocky landing when the pod itself isn't doing much. Unfortunately, this effect is most noticed when the joysticks do not become "active" like they should.
Please understand that your opinion does not represent necessarily the majority. I respect your opinion and don't want to change your mind on your view of the ride, but I know of a lot more people who love MS than don't, so from my reference point I'd say its quite popular. Not to mention that it is attributed as part of the reason for Epcot's continously increasing attendance.