I have a smaller version of this tail bag, the roll bag. I bought it as a set along with a taller, more rectangular bag to use on my Shadow. This bag was designed to attach to the top of the larger bag using 4 short straps. It worked great on the Shadow because I had a luggage rack to attach it to. The quality seems pretty decent, and the set came with a waterproof cover.
However, I do NOT have a rack on the CTX and I'm not sure the Honda one is that useful, because it seems so small. I went to Cycle Gear and got the Sedici Veneto tail bag. I haven't been able to actually use it so far, but I bought it and packed it for a camping trip that I had to cancel (sorry, Bob). The Veneto is amazing because it has so many attachment options--it even includes a pair of bungee cords tucked into a compartment in the bottom of the bag. It has straps that I was able to put under the CTX seat to do most of the holding. The Veneto also has sections that unzip to expand, especially vertically, which made a HUGE difference in its capacity. The Veneto also includes a waterproof cover.
I use the cylindrical MotoCentric roll bag to hold my rain gear, a full set of extra dry clothes compressed in a big Ziploc bag, and a set of 4 REI straps. It's packed full, but is perfect for that stuff and I can stick it on top of the Veneto bag and pull a bungee net over the whole thing. The Veneto even has straps that run up over the Motocentric bag to help hold it on, too. I hope to try it in a few weeks when I will be making another trip to the same area as my previously-planned trip.
Since the CTX1300 top shelter (instead of a gas tank) is plastic I'm not so sure a magnetic tank bag will work unless you do something extra like the ST1100 riders sometimes do. It also has a plastic top shelter. A few with those bikes glued large heavy duty magnets on the underside of the top shelter just where the tank bag magnets would rest. They had to be really careful of the polarity to make sure the tank bag magnets weren't repelled instead.
Got me the Saddleman. Permanent fixture on the pillion. Packed with everything from rain gear to a tire repair kit with mini compressor, jumper cables and a roll up tool bag with enough stuff to do an in frame on the side of the road.