I had a negative experience, but not on the scale of madmoomix... thankfully, but I consider myself lucky considering I took exactly 150mg...
I've never had any negative interactions with MDMA, MXP and DPD IM, or "proper" nitrous use. Nor had I taken anything at all for weeks, not even OTC medication. The only issue I can think of is me having a slight hint of what felt like a cold, like in the stage where you're not even sure you've got one, and I never did get one after. When I get a fever I frequently have nondescript hellish dreams, where I feel utterly dissociated and thoroughly miserable, so perhaps I could have experienced a mix of fever and being pretty high!
I ate nothing for 6+ hours before to prevent nausea (perhaps the cause of the extended trip perhaps?), and took it at 8pm. Had a few hours of the usual "tingly scalp massage" effect, and started getting CEVs at about 11pm, so beautiful that I actually spent a lot of time with my eyes shut (I was in a club at this point, not my kinda music, but it was ok). I'll spare the details of the trip for a full length report... but it was vivid and powerful! EPIC tracers which made crossing the street impossible (car headlights trailed for whole seconds), form constants wheeling about on all light surfaces, objects moving by themselves... the most vivid moment was waiting for my girlfriend to come out of the toilets. The toilets had male and female graffiti style cartoon dinosaurs on the doors, and these started to animate themselves... I must have spend 5 minutes just going "what the shit....". After getting home at 2am (absolutely tripping nuts at this point, psychedelic patterns wheeling about on all light coloured surfaces, epic tracers from light sources) things started to get a bit sinister.
I attempted to sleep, and managed to for brief moments. At some point I descended into my own mind, and had lots of evil visions about my daily life, none of which I could remember particularly well. I do remember becoming convinced that by consuming Bk-2c-b I had managed to destroy the entire universe, which disurbed me greatly. I really relate to people feeling such powerfully negative things that they wonder if they will emerge with thier mind intact... Eventually that passed, and I awoke to find some very unpleasant physical effects.
I started to have strange semi-believeable hallucinations: my partner lying asleep next to me had morphed into some kind of mystical jungle goddess, with rainbow patterned makeup, and I was physically and mentally overwhelmed by her beauty. That was kinda cool, but rather creepily, the bedroom window appeared to morph open and closed in stop-motion, and I distinctly remember thinking "ok, enough, I want off this ride". At this point I've been tripping HARD for 9 hours. The next thing I know (probably after more terrible sleep) I've got black strands and shapes creeping into my vision from the edges, and the entire world went into insane after-image stop motion, so much so I felt on the verge of passing out. My hands felt disproportionately large (even though they were not), and my face and throat felt itchy and swollen, very uncomfortable. I've never had an allergic reaction, but I treat people at work with them very regularly, and that's what I'd imagine one to feel like.
I had a VERY profound and primal sense that I had ingested something immensely "bad" for my body, as if I were a caveman having tried a new plant. My body was screaming at me to get this substance out, I wanted do shit, piss, and vomit, but I had nothing in me to come out, despite being certain that just one retch and the substance might possibly be gone. At one point it all became too much, and in my deluded state started to worry that some sort of evil toxin was in the air, poisoning us both, that's how physically terrible I felt! I actually woke my girlfriend up and asked her what she could see, to make sure she wasn't seeing and feeling the same thing as me. Thankfully she said no, and went to sleep instantly. I didn't dare wake her, only to have her insist I go to A+E, not where I wanted to be.
I felt overwhelmingly strongly that this was my own struggle, to face alone. I wouldn't wish the sense of "evil" I felt on anyone, I refused to let my girlfriend feel the fear I felt. There's a fantastic medical phrase, that someone can feel a "sense of impending doom", when someone says "I feel like I'm going to die", I feel I have a new understanding of that phrase! This "reaction" felt distinctly organic and plant like, as if my body felt it could actually identify the irritating compound. I itched all over, and decided to administer myself some Chloraphenimine orally. Never have I felt such relief... putting two and two together, my body was releasing Histamine for some reason. I deal with medical emergencies for a living, and that unfortunately makes me stubborn to seek medical attention myself, but I didn't deem myself in need of any as best I could tell! Rather foolishly I decided that I didn't want to end up in a resuscitation suite of my own choosing... in my darkest hour I wanted to be alone, perhaps having seen enough people die, I associated that place with death, rather than life.
The physical side died off some 16 hours after dosing, but I was still tripping... the birds outside sounded digitally processed and repeated in an echo, a bit like when you reduce the delay time on an audio delay unit in real time, and get that insane speeding up noise (for the musicians out there). Every window in the house was covered in rain drops, each one of which was refracting rainbow light... I was afraid to drink a glass of water, the sheer beauty of it looked like stress patterns in plastic or glass... I almost feared that I'd somehow obtained a whole glass of pure liquid LSD, and I'd just swallowed a whole mouthful! By this point sleep deprivation is getting to me, and the CEVs were too intense to sleep through. This carried on for pretty much the rest of the next day, and I recovered watching a few films. The CEVs died out after some 23 hours, but it took until 28 hours after dosing for me to stop seeing the pattern on a pair of cammoflage trousers constantly morphing about. My partner noticed that my pupils were at one point dilating and constricting in a pattern, which I've never seen or heard of before!
Psychologically, I feel like I tested the strength of my own mind, and came out on top. In places it was utterly horrific, but nothing negative came of it, in the same way that running a race is painful, and leaves you drained, but you're sort of glad you did it. I destroyed my remaining bk-2C-b, as I don't wish to push my luck again if it were an allergic reaction, but it will be an experience I'll take to my grave, and given that I can't go back and change time, I'm greatful for it. That said, it was actually awesome fun for about 2 hours, standing in a takeaway utterly immersed in a psychedelic wonderland was quite something, but I'll stick to known, tried and tested substances next time! I really did NOT apreciate the duration of my trip. Such a shame, others seem to have enjoyed every last second of their trip with it.