Ketamine is the only psychedelic drug I get cravings for and my total lifetime use has only been around 8 grams (3 racemic, 5 s+) over a 6-8 period in 2007 and 3 grams racemic over a 2-3 month period around a year later.
The first three grams were extremely rewarding, and I was pretty disciplined at first in keeping my usage under control. Then a few months later I had the chance to get 5 grams of s+ for very cheap and ended up using it daily. This year was also initially very rewarding, but I did end up going through it faster than I would have liked.
I can always find when I'm starting to use ketamine too much because rather than listening to music or something with my eyes closed I'll be sitting there surfing the net and doing normal everyday things but in an impaired state.
The sad thing is, I don't think I ever experienced a k-hole. I had a few experiences that felt on the verge of a ++++, but these were always eyes open and I was aware of my surroundings. The best way to describe it is that it felt like consciously waking up in a dream, although I knew I wasn't asleep physically. I'm quite aware on an abstract mental level of the illusory nature of 'reality' and my ego, but these concepts seemed to be becoming experientially real, which was scary enough that I could never quite give in fully to my experience.
I think part of the reason I crave it is because is because of the type of experiences it gives me. I really enjoy psychedelics and I feel that ketamine can be very escapist.
Whereas the classic psychedelics are more like a magnifying glass for my own mental world, ketamine floats me gently to another one.
Another difference that makes it much easier to get 'hooked' on k is that fact that the come up is very non-threatening and enjoyable (likely due to the calming, glutamate inhibiting effect) in comparison to traditional psychedelics, where for me, the come-up is by far the worst part.