It seems that carbogen is not very popular. I'm a little surprised more people haven't tried it. It's not like CO2 and O2 are hard to find.
I've done carbogen a few times recently. I don't have gas breathing equipment, but I made some carbogen the ghetto way. I filled an upside-down plastic storage bin with water and bubbled 3 liters of CO2 and 7 liters of oxygen into it. I breathed the gas through a rubber hose.
As expected, the gas was very unpleasant to breathe. The feeling of suffocation is immediate and it takes a lot of focus and willpower to not take breaths of fresh air. I'm not sure it's fair to call carbogen a panicogen though. I mean, it does cause a panicky feeling, but it seems like more of a physical reaction than a true feeling of fear. It isn't like a bad trip. Relief comes pretty quickly once you start breathing fresh air.
Unfortunately I haven't been able to have a full on hallucinatory experience like the trip reports talk about because of my set up. 10 liters of carbogen is only about 6 breaths. It still is a pretty intense feeling at that dose. You start feeling disoriented and your vision becomes fuzzy and bright, a little bit like when you are out of shape and run for as long as you can, or if you get dizzy upon suddenly standing up. There is a bit of a euphoric feeling, but also a sick feeling. Overall, I'd have to say it feels bad, but not terrifying. It does seem a little bit psychedelic, but really it's more like passing out. The first time I did it I noticed some visual effects that reminded me a bit of a traditional psychedelic, but the most recent time I mostly just felt dizzy. Maybe there is some kind of tolerance effect.
The intense part of the experience is pretty short lived, maybe only a minute or so, but afterwards I feel pretty weak and tired for maybe an hour. During that time I can't help but feel paranoid that I could have damaged my brain or something. I feel completely normal later, but still, using carbogen feels sketchy to me. I probably won't do carbogen again, even though I am still a little curious about what it does at a higher dosage.