I'm an avid fan of SPuDs.
They are easy to clip in and clip out of. (I did try toe clips/straps a few times.. never did get on with them).
They do encourage better pedalling technique.
Compared to flatties, they are faster on a sprint and they also make you tire less quickly on longer rides.
But, when going from flat pedals to SPuDs (or even toe clips and straps for that matter) I think it takes some time to get the full benefit of them. Pedalling (or rather spinning) will improve by some margin straight away, but I think goes onto to improve gradually much more over time.
I think the muscle groups used also need time to retrain. When I first used SPuDs, I was a little disappointed. I've been riding a couple of bikes with normal flatties just recently, including piloting the tandem, and I miss using them. Working so hard on the down stroke on flatties seems so alien to me. I also find it really tiring. And it's really annoying when my feet slip of the pedals!
My stoker got some SPD pedals and shoes for Christmas. Her pedalling technique has improved so much so, we are tearing along the flat on the big ring nearly everywhere we go. And when she puts the effort in, I feel like I'm doing her a real disservice because I can't reciprocate so much by mashing the pedals.
I'm nervous about putting SPDs onto the front of the tandem, but I dare say I will try it sometime soon for a solo ride. My shoes do have a recessed cleat, but after about twenty months of everyday wear, they are about knackered. I can't afford another set right now, so I'm going to use another pair I have but unfortunately the cleat is not recessed into their sole. I don't think a protruding cleat will provide enough grip when I put my feet down to support our tandem - more so in the wintry weather. Whilst we are discussing the virtues of toe clips versus clipless pedals, what does the panel think?