I use it, and I have posted a couple of bugs associated with spatial type support in PL/SQL Developer.
If at least support for Oracle objects would be better... I don't even dream to visualize spatial data in PL/SQL Developer, but at least I would like to be able to edit Oracle spatial type columns.
You can tale a look at Oracle's SQL Developer + GeoRaptor project. It seems to be a promising combination even I do like PL/SQL Developer for non-spatial work with Oracle database.