hi ,
we have been running on ML 11 for 3 weeks , going from ML 8 and have not experienced any problems so far , However it is best to APPLY the patches and see how they run first but before you do this check for applied patches on your system and commit them and then apply ML 11.
Also if you have other s/w running i.e. HACMP you need to upgrade this to the latest level or the level suggested otherwise the upgrade will fail indicating this and when you do an instfix -i | grep -i ml it will show ML 11 but say not all the filesets were ML 11 were found
HTH