(I just cut and pasted my reply to the question in the SIG section, Hope it helps):
The majority consensus on the Pro Mag brand is to avoid them. Most aftermarket mags like ProMag, USA, etc, are not deemed reliable.
With that said, I have several Pro Mag hi-caps for my 228. 3 of them were very reliable from the start. I did get one used one, and it wouldn't keep the slide locked back when empty, and had multiple failures to feed. The mag spring was quite loose and easy to load....In fact I could squeeze 15 rounds into a 13-round mag. I replaced the spring, and it works well. I don't know what it is, but every other pro mag I've owned for different pistols have been unreliable. The SIG 228 and 226 have accepted them with open arms.
The best aftermarket hi-cap mags are made by MecGar...as they were one of the original manufacturers for SIGSauer. Unfortunately, these sell for nearly the same price as factory hi-caps.