Quote from nitro:
I use it and over thousands of automated trades, I have never had a problem.
nitro
I generate thousands of trades per day (maybe more than 10k, I think), and it is pretty good. However, some of the internal stuff is a bit convoluted (and hence a bit slow), so I made a few patches. However, as I am not a core contributor to QuickFIX, the patches are passed to some of the core developers.