Pretty much all recent Logitech 'high end' mice I've used have had really shitty middle-clicks. It's because they usually had this free-spin option to the wheel, which is all well and good, but if you like to use your middle-click a lot, I'd probably be looking elsewhere. Definitely don't get one if you do any 3D work.
F'n oath. The free spin option can be disabled in Setpoint I believe, but then you've got another problem, namely that the abomination called Setpoint is installed on your machine.
Such a simple mouse in that regard, but you'll pry my Logitech MX518 out of my cold dead hand.