We have been using Suhosin since it was released. It has never caused a problem. The only PHP errors we encounter are coding errors. However, we do not use third party PHP on our website, so I cannot comment on whether or not it causes problems for other people's code. Having said that, we try very hard to code in a very strict fashion and take security very seriously. Our background is in C/C++ coding, so we apply many of the best practices for coding in that environment to our PHP code.