as I only shooted it, I had a pretty short trip (1 hour or so) but it blew my mind away.... this shit hits you right in your face like a rolling bus or so, than there is nothing, you don't even know if the injection went well, so paranoia may occure. then you try to puzzle together what has happened to you, why you are completely dissociated from yourself and your environment, some people may enjoy it, I don't know. haven't tried it too often yet. but I had several very interesting thought patterns during the trip, unfortunately I forgot almost everything. when you get calm and feel comfortable and secure, you may enjoy the strange buzz, you may find silently playing music attractive or just laying around and laughing being completely away in the k-hole or 4th room. while under the influence of K you may experience an easy intense breathing experience (this is normal, nothing to worry). you should always remember that the effects are temporarily and don't last too long.
don't forget that ketamin is an anesthetic drug, it puts you in kind of sleep mode, so sit/lay down or try not to run around at least. you can also experiment with very small amounts, this I found pretty interesting, but the effects don't last long then.
don't mind your negativ dxm experiences, I wouldn't wanna do dex eather again.
after your K anesthesia wears off you may feel full of energy and excitement, you may even feel a mood-lift on the days after your trip. So have a pleasent flight with Ketamin airlines and don't forget to tell about your experience!
one more hint, I don't know if this is also the case when you snort the powder, but the drug stabilizes your cardiac cycle so if you take the trip on midnight you might find it difficult to sleep afterwards, just like having a stimulant but without the euphoria.
concerning the similarities to dxm, I'm not sure. when I did dxm I always smoked weed and drank also alcohol, I only remeber the experience being on dxm as rather unpleasent/irritating.
concerning the dosis, depending on your weight, I would start with 30 - 50 mg as a low dose just to test it, 100 mg would be a stronger dose that will keep you busy for longer.