I do use Mellel and it feels snappy and trustable, unlike Word. I keep Word around too, though, both for compatibility's sake, and because I have to concede it does do some things better, like tables and cross-references.

Note that Mellel and Bookends usually are available for a special discount if bought at the same time. They are also programmed to work well together.

I should add that working in Mellel pretty much requires you to think in terms of styles rather than ad-hoc formatting changes. If this is new territory there may be a bit of a learning curve. But if you're talking about a dissertation-length project, IMHO you'd be crazy not to employ style definitions no matter what word-processor you use.