Developer and Designer are different tools!
Although Forms and Reports can be generated with Designer, it is not the same as Developer.
Designer can be used as a rapid application development tool, but it is mostly used as a CASE tool.
Unless you are willing to commit a lot of time to learning it, don't bother.
I personally like the tool, but was lucky enough to receive two Oracle Instructor Led Training courses in 1998. In addition, I have slugged through a book or two.
Good luck.
David Knight
OCP DBA 8i, 9i, 10g