In the interest of full disclosure, I think tail bags are a good thing but I really don't like the looks of any of the hard ones. I just think they look awkward on an FJR. I do like having the extra cargo room, mostly for spur-of-the-moment store runs, so I use an expandable soft case from my old KLR mounted on a Garauld rack. Of course, it doesn't lock, and isn't waterproof. A garbage bag inside the tail bag takes care of water. As to the worry about theft, well, keep the high-value stuff in a side case or carry your tank back inside, I guess. I haven't had a problem.
For camping, I add a rubberized duffel I got at REI on the passenger seat. Frankly, every time I get ready to leave on a long trip and look at the HUGE pile of crap I have to load on the bike, I really can't believe it's all going to fit. But it does. And don't use bungees. Get yourself some
Rok Straps (notice I keep a pair crossed over the passenger seat all the time. They come in handy a LOT.)
I have the backrest that came with the Garauld rack, but almost never put it on. For one thing, the bag itself acts as a backrest if I have a passenger. Also, the KLR bag I use needs more room to sit flush on the rack, so the backrest would be in the way. Easy to remount if I want to though.