My son-in-law had over 25,000 miles on the same model 105 8-speed levers when he upgraded to 9-speed and the 105's were still working fine.
My son has 15,000 miles on a set of RSX 7-speed STI levers (the bottom of Shimano's line at the time) and they still function perfectly.
The few problems I've heard of with STI's have been gradual stickiness and reluctant shifting, not catastrophic failure.
I ride with both STIs and barcons. STI's are FAR better.