For the price Formula cogs seem to be fairly well made and I have had no issues with them... I always check the threading on cogs and hubs as even the best of them can make errors.

I have run Formula, DA, Sub 11, Surly, and some vintage cogs and not had any issues with any of them... stripped threads are usually a result of a poorly installed cog / lockring or a combination of this and really poor quality parts.

If your lockring fails and that cog comes loose it can generate a great deal of shearing force to the hub threads... it is pretty much essential to own a lockring tool and regularly check that things are snug.

And like Op said... for 25.00 the DA cogs are a no brainer.