justbreathe: M-CAT...
Bear in mind it's a relatively new drug, and it's illegal, so a lot of the 'information' is actually anecdotal. I spoke to a drugs agency a couple of times, who said not much is yet known about it.
M-cat is not particularly a club drug. It was a 'legal high' until very recently, so easy to get hold of (sold as 'plant food'). It used to be very cheap, but it's got more expensive since it was made illegal. My DS (it turns out - I didn't know at the time) first had it when he was 13, given to him by 15yo friends. Round here it's very, very widely available and even 'nice middle class problem-free' kids 
have tried it.
People I've spoken to about it (including my DS and a couple of adult friends who have tried it) report a range of responses from "it's nice" to "yeah, it's alright" to "it doesn't do much for me" to "it's horrible stuff". I guess its quality varies widely because it is mixed differently. It is snorted.
While a person is actually taking it, it seems to be a euphoric - people have told me it's a 'party drug' like E. But its effects are very short-lived - just minutes. It is very, very compulsive and hard to moderate: once you start taking it, you can't stop until it has all run out, and if someone comes along and offers you more, you'll take it. I found out about my DS's m-cat use because he took £90's-worth in one evening, and gave himself sweats and palpitations that made him think he might die, so he woke me up for help. :(
It is also a dis-inhibitor, like alcohol. I have seen kids on it do very stupid things on train tracks and railway bridges; but more seriously, some kids have done illegal things like burgle houses and steal :(
Medically, it is a stimulant. It increases heart rate, pulse and often respiratory rate too. A couple of people do seem to have died taking it, from heart failure.
When a person is getting it out of their system the next day, their body reacts pretty much as it would to other nasty toxins: sweating, flushed cheeks and chest like a viral rash, mild diarrhoea or a dodgy tummy, a lot of sleeping... It's hard to tell this from alcohol or other drugs of course, but I picked up one or two 'tell-tale signs': most kids seem to get 'glassy eyes' and my son also gets flushed pink cheeks.. Users also smell really funny - sort of citrusy but unpleasant - like toilet cleaner and oranges. 
(My son used to come home early in the morning and jump straight into the shower to try to hide that). I also suspect dramatic mood swings and aggression following an m-cat 'binge' - but I have to admit I can't be sure whether that was the m-cat or the skunk or the combination of the two.
For my son, it was quite disastrous (one or the other or both). The compulsive/dis-inhibiting effects were worst. It was so compulsive/he got such a habit that he stole hundreds of pounds from me, and sold many of his possessions, and some of mine and his brothers. He also committed one burglary - not for the money, but taking an item from a friend's house because "it seemed like a good idea at the time". :( :( He was also short-fused and violent on a couple of occasions. I was also worried (though I knew it wasn't a big risk) of him dropping dead of a heart attack.
The reason I thought it might be worth making you aware is that my son's very worst behaviour coincided with heavy (more than weekly) m-cat use. Also, his skunk smoking increased while he was doing m-cat (I think because the two drugs 'balanced' each other in some respects) so the skunk-related problems were worse too. He was completely illogical and irrational when he was on it - and the kind of 'losing the plot' craziness you describe was characteristic. So it's worth considering...