Okie, a word about hot plates (Halacha, though V’Nishmartem trumps all)
All off the top of my head, do not be somech on l’maaseh
-R’ Moshe holds that a hotplate without any knobs has a din of kli al gabey kli, hence one could put food on it on shabbos without worrying about nirah k’mavashel. Of course, regular bishul would apply (fully cooked, no liquid). Most Poskim disagree with R’ Moshe, and require regular kli al gabey kli.
-However, many Poskim – R’ Shlomo Zalman amoung them, say that such a hotplate does not have a din of kli al gabey kli, but does have a din of garuf v’katum. Hence, once could leave food on it – before shabbos – directly on the hotplate ( but not put it on the hotplate on shabbos itself) and return it on shabbos. This also is an issue of great debate.
-The water hotplate thingy attempts to replicate kli al gabey kli by saying that you are cooking the water within, hence putting something on the blech is kli al gabey kli. R’ Elyashiv rejects this on the premise that kli al gabey kli is only when you actually plan on eating the first dish. I believe that R’ Yisrael Belsky rejects this blech as well, but I am not sure. Of course, it’s not worse then any other blech, if you put another kli on it, you could put a kli. Presumably poskim permit it’s usage, or else it wouldn’t sell. But who knows.
I am intentionally using halachic terms. Ask your LOR.