check out
http://www.erowid.org/chemicals/ketamine/ketamine.shtml
They have a wealth of info on ket, including the effects of long term use. There has been research to show that subanesthetic (i.e. reecreational) doses of disassociatives (including Ket and DXM) is very hard on your brain, but then again there has been research that's shown ket to be less harmeful than DXM or PCP. Ket can cause psychological dependance, and some long term users report parinoid delusions and ego-centric thoughts (I.e. there is a global world conspericy against ME). bottom line, everything in moderation.
Information = Freedom