I have had adequate results with store-brand SPD copies from Nashbar. They work fine and you can get them for around $20. I took one apart after about a year of occasional use, and found a bit of pitting on the spindle, but I am 220 and a fairly strong pedaler.

I also liked the way crank bros. smarties work but one of mine developed a fault after I had had it for a while, and there's no service available. I think I may be able to repair it with a hardware store e-clip, but it did go bad.