[Daniel Andreas]"I have rented it once before and there was some issues getting all the settings right betwen the 1394 Settings on the HDX and the settings on the Firestore. "
There are not issues, there are RIGHT and WRONG/INCOMPATIBLE settings. You have to learn what works. Instructions have to be followed
[Daniel Andreas]"I also heard from some operators that the device is temperamental -- any insight is much appreciated as I am about to purchase this beast. "
I, for one, will only use the FS-100 with locked down cameras. This is a spinning HD so I don't trust it for run and gun shooting, others may.
Learn the device inside and out and it should do fine for you. My 100GB FS-100 is harding being used now that larger cards are out. But for long concerts on a tripod it is price new to work with.
I have been using the FS-100 (100gig) version with my HDX900 for almost 2 years now. To answer your question, the firestore drive is tempermental and not always 100% reliable, but the beauty with the HDX900 is that you can roll tape simultaniously with the FS-100. For anything important, I never run it without a tape as backup.