The best way to get data out of DSpace is use to use a protocol called OAI-PMH.
It's a rather old protocol and slightly awkward to work with, but we provide a command-line tool that wraps most of the odd bits so you can focus on just asking for the data you want.
MIT Libraries maintains a command-line tool that we use internally to harvest records from various sources. The tool is available publicly so you can use it too.