I have a report on an experience with mCPP and meOPP up on Erowid as well as the trip reports forum. At the time I raved about it and compared it to MDMA, saying it was quite nearly the same thing (without the whole empathy, of course, I thought it was about a 4 out of 5 -- 5 being ecstasy). After a while of thinking and retrospecting about the experience (and after having some more common psychedelics in the meantime), I would say that it's closer to a 2.5 or 3 out of 5 on that scale. meOPP is much weaker compared to mCPP, so I usually do both at the same time -- meOPP is not very spectacular on its own.
Really, as far as I have seen with the piperazines I've tried, I've had better, more flashy experiences with other things. The euphoria from mCPP is pretty damn good, but if alternatives are available, there's no reason to take mCPP (in my humble opinion). The psychedelic effects are weak, but you can have an enjoyable time with them in the right setting. The only thing unique going for these drugs are the fact that they are legal and easily ordered, even in the United States (for now), and the comedowns are not severe at all (I didn't even get the headache that people talk about).