I think its like all illegal drugs, physically its as bad for you as whatever it is cut with.   Mentally its harder to say what the long term effects are, i certainly doubt it is good for you but recreational use is unlikely to be disastrous.  From personal experience i would say there is definitely a period of space-cadetness for up to a week after depending on quantity and quality of what has been consumed.  I do know folk that have gone on regular benders on the things and there is no doubt in my mind that they have been adversely effected by this.  Jumpier and a  shorter attention span being the noticeable traits.