This is such a handy little expression, I thought I'd highlight it in this post. mea sententia - it is in the ablative case, so both final a's are long - means in my opinion (IMHO). If you are speaking Latin and you don't want it to sound like everything you say is a declaration of truth, then mea sententia is a godsend. Moreover, it doesn't even muck up the grammar, by beginning an indirect statement or anything of that sort. Go ahead - use it!
Marcus is holding such instruments because he is a painter. Marcellus, in my opinion, is an excellent painter.