No. Cactus spines are not poisonous and if they pierce the skin should - provided they are removed promptly - result in no ill effects. Obviously and cut or piercing of the skin can result in infection and good hygiene practices should be implemented as a matter of course. Some of the 'other succulents' most noticeably Euphorbias do require greater respect however. Their milky coloured sap can be extremely irritating and skin contact avoided if at all possible. Contact with the eyes, mouth or other sensitive areas must be avoided under all circumstances.