I've used the Carradice SQR post bag. Bit awkward to fit with a lot of height needed. Also bit awkward for leg clearance depending on how much layback your running.
It's a nice enough bag though and the latch system although looking a bit home made does seem to work well enough and provide a secure mounting. Like most carradice stuff it's kinda waterproof but you do need to have things wrapped inside to make sure it's protected from a soaking down pour.
Depends on what you need to carry really ? It all seems to add up and it soon ends up easier to go with light rack set up vs loading a race bike with bags way up high that throw of the handling and do not offer that much capacity for actually carry stuff you might need + food / clothing options... But if you've toured already I guess you will have a good handle on what you need to stow.
Have a look at the user review of the revelate stuff : http://tramplite.com/2013/03/revelate-b ... eview.html
I'd certainly go with this option if doing light touring and aiming to keep carried weight to around 5-8kg all in.