The only truly bad vet, in my opinion, is one who suggests you put your beloved animal to sleep (barring any life-threatening issues). My vet did not suggestion I put him to sleep, however MANY vets do so or comply with owners who decide they don't want to bother because it's "just a pet." Vets who recommend putting a diabetic animal to sleep should immediately be disbarred (or whatever they do to vets).
Before I start "vet bashing," I want to add the caveat that most vets are general practitioners, and not just for one species. They have to know about dogs and cats and birds and small animals and reptiles and everything else in between. And when people bring their animals into the vet, it's usually for something minor (like shots) or severe (like DKA). In many cases, cats aren't diagnosed with diabetes until they're already in kidney failure or DKA, not because of the vet, but because cats are very good at hiding pain and sickness until it's gotten that severe. Vets know how to treat a DKA and kidney failure but they don't necessarily know how to treat chronic illnesses like diabetes, much less across different species.
In June, 2010, this article was published in the Journal of the American Animal Hospital Association. It spells out clearly that wet food, (ultra)low carb diets are a must. But vets proceed to sell EXTREMELY HIGH CARB (usually DRY) "Prescription Diets" from Hills, Purina, or Royal Canin anyway because they don't know any better! Why? Because vets received very little (if any) nutritional training. What training they do receive is provided by the pet food companies. So, of course they're going to think that Hills Prescription Dry M/D (what my vet sold me initially) is good for diabetic cats. The wet alone has 14% carbs and dry always has WAY MORE carbs than wet. ALL CATS NEED LESS THAN 12% carbs, diabetics even more so (10% or lower). The pet food companies are basically soft-killing our cats with the re-purposed dog food with added "cat nutrients" that they're claiming is "good" for them. It is amazing to see cats join the FDMB board and when first diagnosed, they're so high that the meters read "HI" (this is over 500 on most meters, I believe). After a couple of weeks on low carb, wet food, many of them are reading NORMAL ranges (i.e. ~40-130) and some are already in remission!
The other problem is that vets will often think cats and dogs are alike in diabetes treatment. They are VERY DIFFERENT! Cats are carnivores, dogs are like humans and are omnivores. Vets mistakenly prescribe insulin that is too harsh and causes too big and fast a drop in cats (i.e. humulin, vetsulin), which will work fine in dogs (NPH, Lente). That same AAHA article states quite clearly that the only recommended insulins for cats are Glargine (i.e. Lantus or Levemir) or PZI. Again, this goes back to vets treating numerous species for numerous problems. Information from 3 years ago is not long in terms of information for a vet to keep up on, so many of them are outdated simply because it's impossible to know everything about every animal whenever it's first published.
One more problem is not the vets themselves but the PET OWNERS. When an owner is initially told that, "Hey, guess what? Your pet has diabetes. And then also guess what? You have to give him 2 shots a day for the rest of their lives." A lot of owners say, "Eh, it's just an animal. Time to PTS and get a new one." (You would be surprised how many people have told me I should put Michelangelo to sleep. He is a regular kitten that you can't even tell has diabetes and is actually healthier than most other cats because he's been fed wet, low carb food for most of his life.) If the vet then recommends home-testing on top of a diabetes diagnosis, I'm sure that even more owners would choose to PTS. Most good vets, however, will not PTS a diabetic animal, but will instead have the owner surrender the animal and turn the pet into a shelter (where they'll eventually be PTS anyway because no one wants to adopt a diabetic animal...or do they?)
1 through 3 are not necessarily signs your vet is bad; just that he is out-dated or uninformed or even misinformed. Numbers 4 & 5 are the ones that show the vet is close-minded, egotistical, and willfully ignorant and does not have your pet's best interest at heart. That's when you should start looking for a new vet.
If your vet is open-minded, willing to work with you, and requests to look at the information you've found, then you may just have a good vet. In my case, my vet asked if I could email him all the links I'd found online and even asked if he could use me as a reference for any of his other diabetic cat owners that might be interested in testing and home monitoring. So, just because my vet started out selling me dry prescription food and not recommending home-testing, he turned out to be quite a keeper.
Footnote: An excerpt from the 12% carbs study mentioned above (emphasis mine) and the reason why ALL cats should not be feed dry or higher carb food even if they're in remission or don't have diabetes in the first place:
The prey-based natural diet of a hypercarnivore such as the cat supplies insufficient carbohydrate to meet the metabolic demands for glucose required, for example, by the brain, and this demand for glucose is met by a high capacity for gluconeogenesis from amino acids (Eisert 2011). Since the cat appears to be metabolically adapted to meet its glucose requirements on a very low carbohydrate diet, it seems unlikely that the higher carbohydrate intakes seen in the present experiments is the result of cats actively seeking higher carbohydrate intake, although this cannot be completely discounted.