I've only had two diabetic cats, so my experience is probably limited compared to some other posters, but this is how it went for my Toby.
He started to have difficulty using his hind legs about 4-5 years ago. I took him to the vet 3-4 times and he decided it was some kind of neurological damage and was probably not treatable. As it worsened I requested a referral to a specialist, who after watching him walk for about 5 seconds announced it was diabetes. (Original vet was so certain it was neurological that he didn't even do bloodwork) His glucose was around 450.
Went back to the original vet where we did the curves, etc, and ended up putting him on insulin shots twice a day. I put him on Purina DM, both dry and wet. The vet told me he'd have to receive shots the rest of his life.
Toby started out at 20 pounds and is now 12-13. He hasn't had a shot in about a year and a half after taking them for three years.
I did eventually switch vets for Toby and the new one was very confident that with weight loss and proper diet we would get him off insulin, which we did. Although I continue to feed a combination of Purina DM wet and dry, this vet is also a firm believer in an all wet diet for all cats.