Psychedelic thought loops can be extremely psychedelic, although they are also probably the most difficult psychedelic experience to make sense of or handle. For me, the thought loop can be of two types: an anxiety/neurotic thought that won't go away, or the "god-loop" (just made that up), where your thoughts lopp around and around and it seems to be a continual awakening from your life into a higher state of awareness, eventually leading up to re-realizing that you're the universal consciousness and that everything is an illusion.
The first kind of loop I've only had on one psychedelic, which was mushrooms, several times. Those ones are no fun, as they only lead you into panic and irrational behavior. You can also get into a neurotic loop without drugs.
The god-loop, however, is what has led to most of my most profound and memorable experiences. I've gotten into this loop with mushrooms, 2C-E, and 4-AcO-DMT, and so far nothing else. The time it happened with mushrooms was the gentlest and most beautiful, and left me the most calm. The time with 2C-E was the most terrifying and mind-shattering experience of my life; I hadn't had it happen like that before and I thought I was dying and would be swept away forever in the flow of everything. The time with 4-AcO-DMT was frightening to the extreme, but I felt buffered somehowand was able to keep my fear to myself and ride it out and eventually reach godhood (for lack of a better term). The time with 2C-C, I spent an hour or so (the length of time the loop lasted) pacing around my apartment frantically, contemplating 911 and suicide.