I came into microstock sites after trying to figure out what to do with all my outtakes from other shoots, rather than thinking I would make a lot of money.
I've not uploaded every image I can find, but ones I thought would be useful.
Three of the first places I joined a year ago have done nothing: one folded (JustMacros).
One was still there the last I looked. They approached me and asked for some full size samples they had seen on my website. They sent me a photographer's package which looked perfectly legit - even had my lawyer check it out before i signed and sent it in. But, they took my images and developed no user interface or account for the photographers (and they are not just my images there) - there's no way to check on anything there. They made big promises and carried through on none of them. For all I know, they've been using my pics themselves (they also have a webhosting site and webdevelopment site) - I've even emailed them repeatedly to have my images removed from the site, but after a few months they no longer reply at all and i can't get my images out of there. Bah. (Bestphotosfx).
The third is a site you need to pay (I think it was $50 for lifetime membership) which is really a specialty site that provides images of cities and towns and I haven't sold anything there either (that being said, i did all the appropriate research and legwork and asked a million questions before i signed up - they are legit and do make sales, but i suspect my location is not going to be a big seller). They have good quality images too, and the photog gets $25 per sale (if and when you make any) - Citypix.ca.
I did sign up at Canstock a long while back but submited only half a dozen - two of which they rejected (graffiti on a highway underpass) for "copyright infringement" (can't copyright an illegal work, and defacing government property is illegal - at least here). At that point I thought that perhaps it wasn't worth the effort for .10 - those images have in fact been used in a lot of other places.
For months after those flops I stayed away from it - getting hired to shoot concerts and other similar events and portraits is enough to suit my retired lifestyle.
But, I have a friend who signed up at a few of the micro sites and she talked me into submitting to SS. It's doing okay, though i don't work very hard at it. I've got a small gallery compared to many.
I've also signed up with TotallyPhotos (just recently, so no sales, but very good site so i hope it takes off); Gimmestock (which is also going nowhere and slow to approve images); Crestock (just this week and they are new) and two others that are also just starting out. Except for Gimme and SS the others all sell the images for a base price ($10) and the photog gets a better cut...which is why I am hoping they go somewhere. One of the new sites has both RF and RM images and my uploads on that site are split between both categories. It's canadian so i'd really like to see it take off also.
The best boost to my photography business hasn't come from microstock sites, but from a 'free' site. I've received a number of magazines, advertisements, flyers, posters, etc. in the mail from people who have used my images from that free site (morgueFile). The hardcopies are a great addition to a professional portfolio and they came from images I wouldn't have used anywhere else. In fact, one was from an image rejected somewhere else

because it wasn't suitable for stock....what do they know
My most recent foray into microstock is only a few months old, so it's rather hard to form a solid opinion yet. After SS I decided that I would not like to settle for .20 an image - though they sell very well on SS and i am still submitting there.
I think that photographers who didn't start out in the stock world first have to learn how to get their heads around a different way of doing things; too many get angry when images are rejected. One thing I've learned: every site has their own client base and their own needs (real or imagined) with respect to the types of images they want. I've seen some really superb shots from pros get rejected because they aren't stock oriented. And there's the thing...the difference between stock and art. Not that all art shots won't sell as stock, but many of them aren't suitable and learning the difference is something you have to do or you'll always end up frustrated
One other recommendation: know your work. If you aren't confident in the quality of your work keep shooting til you are. If you have confidence in your abilities and your work, rejections from micros don't mean a heck of a lot.