Mellit is herbal, not homeopathic. I'm a 'lay' [non- professionally-trained/certified] homeopath myself, so obviously I think homeopathy is useful; but Mellit isn't, strictly speaking, homeopathic.
We just got our bottle of Mellit today (Boo was only diagnosed last week), and we will be very slowly be introducing it into Boo's food, while holding the insulin level steady (for now) and watching the BG carefully. All of the ingredients are recognized BG-regulating herbs in human naturopathic medicine, so I see no reason that at least some of them wouldn't help cats -- though the effect would almost certainly be more pronounced with cats who have a slow-onset 'type-II'-style diabetes situation. The herbs in the mixture don't directly decrease BG the way insulin does, but help the body to regulate it and increase the sensitivity to insulin in insulin-resistant persons. (We believe Boo has a degree of underlying insulin resistance, given his remarkably fat physique.)
I'll update the FDMB on any value we find in Mellit as we go along, as well as post on our complementary homeopathic protocol at a later time. (Homeopathy's strength, in the case of diabetes, is its ability to assist in organ and tissue regeneration over a period of time.)