Definitely get fast passes. A DAS is never guaranteed so on the off chance you could not obtain one, you'd have your fast passes.
It is quite easy to use them in conjunction. Remember, DAS is not immediate access. It will give you a time to return based on the current wait time (less 10 minutes). Note: the posted time at the attraction does not always match the time in the DAS system. I don't know why but our last trip we had several that didn't really match. Not that we minded, we just got went with whatever they gave us (it was always longer than posted).
Remember - you will still be waiting, just not in the line. You can get a return time for one attraction while using your fast pass for another.