I had a set of flat/spd pedals. Since I'm almost exclusively riding in SPDs the flat side was a waste of space and made it more annoying to clip into. They were ok, but I prefer the double-sided SPD pedals.
I've had single sided SPD's too. If you want SPD's that you can use with normal shoes then get the double sided ones that have a platform outer surface (I also have some of these) you'll find they're not as nice to use with flat shoes as normal platforms but having to turn the pedal over all the time to get the right side is really annoying and thats one of the best things about riding platforms or spds.
You can also get platforms that clip into SPD's. They're kinda useful as you can remove them. I've used them for a bit but they're not the easiest thing to clip in and out.
I've had single sided SPD's too. If you want SPD's that you can use with normal shoes then get the double sided ones that have a platform outer surface (I also have some of these) you'll find they're not as nice to use with flat shoes as normal platforms but having to turn the pedal over all the time to get the right side is really annoying and thats one of the best things about riding platforms or spds.
You can also get platforms that clip into SPD's. They're kinda useful as you can remove them. I've used them for a bit but they're not the easiest thing to clip in and out.