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Unlike the others I don't mind the PD-M424. I had them on my commuter which I use with MTB SPD shoes when commuting and with flat shoes for nipping to the shops or 5-a-side. Haven't had any problems with them in flat shoes.
Recently replaced them with PD-T400 (Click'R) as I'd buggered one side of the M424s by consistently ramming the bike away in the shed against the pedal (they lasted years of this abuse). The PD-T400's are still SPD but with built in reflectors so legal at night. Heavier and much much looser than the M424s but haven't had an unclip moment on my commute and I'm fairly brutal with them.
I'd rather stab myself repeatedly in the face with a rusty spoon than go back to any kind of single-sided SPDs (like A-530s or the Wellgo equivalents).
If you don't care about flat shoes then M520 are cheap and entry level. I use them (or M540s) on my Audax bikes so they've been good for thousands of miles.
Recommendation for an entry level MTB clipless pedal? Cheap preferred...