i use neither animations nor transitions. but i do extensively use movies (which have a horrible support in Impress), and i use lots of equations (which you can enable using third-party plugins in Impress, but working with them is very difficult).
i've been bitten twice when i've been traveling on a conference and had to quickly put up slides in Impress, and ended up not being able to do what i wanted because of all of its limitations. i ended up using reveal.js, but that also has its own drawbacks, e.g., the lack of UI, which i can use to quickly fine-tune arrows, text positions etc.