I tried several ways to patch panniers and dry bags (and tents). Duct tape does work well but there are better options (unless the tear is really large, in which case duct tape is the only way). But first, the best advice I want to give is when you patch any kind of material, always put a patch on each side of the material. It makes the seal much more efficient and will last much longer. Duct tape is cheap and makes a really good temporary fix. However, duct tape is not good at resisting high temperatures from the sun alternating with heavy rain (I learned from a bad experience in Arizona). Over time, the glue sort of melts and the tape starts to peel off, leaving a terrible gooey layer (this is not very nice on the inside of the bag).
That being said, I always carry some duct tape with me (you can fix pretty much anything with duct tape!). Obviously, a whole dispenser is way too heavy, so I carry the Handy Duct Tape manufactured by Coghlan's
But when it comes to patching my panniers or my tent, I much prefer a nice and clean fix that will hold very well in the long run. For that matter, I use either the Seam Grip glue (hopefully available soon at CycloCamping.com) and any kind of heavy duty waterproof patches or the Ortlieb Classic Fabric Patch Kit (PD620)
. The trick is to use two patches on each side of the material that needs to be fixed, to spread a good amount of glue (don't be shy), then to put these three layers between two books and let the whole thing sit overnight under something really heavy (I like to put it underneath one foot of the bed before I go to sleep so that it's ready to go in the morning). I used this technique on many different materials some years ago and the patches are still as strong and as waterproof as they were on the very first day. It works on vinyl, nylon, cordura, and polyester (PU or PVC coated).
I tried the Ortlieb Patch kit and it works just as good. The advantage of the patch kit is that the material used is really the perfect quality - not too light (like the patches found in most tent repair kits) and not too heavy (good enough for tents but not for panniers or dry bags). Plus, the kit is nice and compact and has just what you need so that you don't have to carry a whole tube of Seam Grip glue.
Here is a good selection of repair kits and patches
for all budgets.