I've pushed things quite far with 25C, with the highest dose being 6mg insufflated, 3mg in each nostril. I have to say, once you get up into doses of that range, 25C gains a level of depth that is astounding. The visuals also become very intense, with full-on morphing of one object into something totally different. For example, a poster hanging on the wall of a female singer standing at a microphone became a human with the head of a crow smoking a cigarette on a background of a sunset over a desert.
My train of thought became extremely bizarre and free-associating, with extremely similar thought patterns to what I would experience when under a medium dose of LSD. There was also a very entactogenic quality to the experience, which reminded me very much of the feeling of aMT. The experience ran for a solid 8 hours, with effects dropping off around this point very rapidly. There was a very mild after-glow accompanied by very subtle visual effects that continued throughout the day (I had dosed at 12AM, with effects dropping off around 8AM) until I slept at around T+24.
All in all, I actually find 25C to be among my favourite psychedelics, with doses of 500μg-1mg being very mild, clear-headed, easy experiences with very mild jaw clenching as the only real outstanding body effect, all in all very well suited to social situations. On the other hand, higher doses in the range of 4mg+ have led to some very deep, profound, ego-obliterating trips with amazingly powerful visuals and deep, introspective thought processes that seem to grow and feed of eachother.
Keep in mind, I don't recommend taking doses this high. Beyond the fact that they are amazingly powerful experiences, the NBOMe series has caused several deaths, and the theraputic index is totally unknown. Furthermore, it seems that individuals vary widely in their reactions to these particular compounds, so caution is definitely in order. I worked my way up to this dose level extremely slowly, watching for signs of trouble and waiting long enough to be sure that my tolerance was at baseline before dosing again, so I could build an accurate picture of my reaction to these compounds. Anyone else should absolutely do the same before even thinking of attempting to dose at this level.