SemTalk
I work on daily basis with Microsoft tools like most of you. Of course a modeling tool like Visio is not enough to do serious business process modeling. You need repositories, syntax and semantic consistency check, more than one modeling notation and method, evaluation functionalities, interfaces to other BPM tools and so on.
I work with SemTalk (www.semtalk.com), which is Visio-based and thus completely integrated with MS Office. Try it yourself.