The unit is pretty good with some nice, sparkly lights. However, I have used it with an API 512c and it does not fit correctly as I can only screw in one of the screws to hold it in place. The 500 series unit still works perfectly though so it's not such a big issue and a lot cheaper than the API Lunchbox for those of us who just want a couple of 500 series units.
I haven't tried it with any other 500 series units but I presume the Fredenstein units would fit perfectly.
Overall I am happy with the unit but the fact that the API 512c does not fit correctly is a bit annoying but, hopefully, will not cause any cosmetic damage to the API 512c.