I have been riding with the Pioneer left side for couple of months now and it has worked perfectly. I have had zero issues with calibration, connections to my Garmin Edge 1000, or connections to my PC based ANT I use for inside training rides. So far I haven't noticed any significant differences in power readings between my bikes that have Stages PMs but I haven't really studied the data closely. The Pioneer factory warranty is 2 years vs 1 year for Stages. Round trip from Portland, OR to San Diego to get the PM installed on my left crank arm was 7 working days.
I bought this kit to retrofit a crankset I already owned, rather than shelling out for a new crankset. The sensor paired quickly with my Wahoo head unit. Relying on left-leg power data does make it somewhat hard to maintain a steady level of effort, and you might lose some accuracy if you're right-leg dominant. However, this option is fine for a basic and economical training tool...One caution my kit contained outdated shipping instructions, so UPS bounced it back to me after 2 weeks as undeliverable. Pioneer's service rep helped me resolve this, but you might want to contact Pioneer in advance to confirm their address and avoid the hassle.