STIs (Tiagras no less) for almost five years, almost 50,000km and nary a problem. Just to balance the debate, you understand, I need to point out a few things. Shifting three gears at once on STIs is entirely possible, in fact easy, with just a single flick of the lever. In-line barrel adjusters (on the downtube or adjacent to the levers) can overcome wear and some damage problems (and I haven't damaged a rear der on an LD event yet).
Mounting a suitable sized handlebar bag is not a problem (I use the Topeak Tour Guide small model that discourages overloading), the cables don't interefere with a light that is mounted at crown or lower level where the beam is most effective in identifying road undulations, and the real advantage is being able to shift while standing to pedal and while my hands are on the hoods (I don't stand to climb on the drops). Came in very handy climbing Sunwapta Pass in the Rockies this weekend.
By the way, one thing to watch for that can trick riders and less aware mechanics... STIs can give the impression of "packing up" with stiff or even seized shifting. The culprit may not be the shifter innards at all, but an accumulation of sports/energy drink drippings on the bottom bracket cable guide that causes the cable to seize. This happened today after a bike that I serviced was test ridden by the owner, who then complained it wouldn't shift on the rear cogs. A squirt of water and away the rider went happy as can be. I have found occasionally that on a long event, the shifting slowly deteriorates, with rear cogs not shifting down the cluster crisply, and the front shifter becoming so stiff as to be virtually inoperable. A sticky cable guide is the culprit.