Sendero:
I shot the Gen 3 as a duty gun (.40 cal) for 7 years. Put thousands of rounds through it with no failures, ever. We switched to Gen 4 18 months ago (and I bought my Gen 3 from the dept.... can't give up an old friend).
Have not fired the Gen 4 as much yet, probably about 1000 rounds so far. But it's performing exactly the same. In fact, I can't tell the difference. I know what the upgrades are, but in terms of performance, I have felt no difference whatsoever. Still dead nuts accurate trigger feels the same and it's as reliable as gravity.
That said, the Gen 3's had trouble with non-Glock weapons-mounted lights. Our department had several instances of Gen 3's with lights jamming at training. I'll note that the lights that were giving problems were all personally-purchased and were generally the 'cheap' ones. My Surefire gave no problems and continues to give no problems on my Gen 4. Those with Glock Brand lights had no issues at all. The Gen 4's apparently solved the 'jamming with aftermarket lights' problem. But you may want to do some more research on that. I think there is some conflicting opinion on the blogosphere. I can only say that the light problem never manifested itself on MY Gen 3. But I did see it on other Gen 3's. And with Gen 4... the problem seems to be gone.
Last... there are lots of guns I love for their workmanship, beauty, history, etc. The 1911 I carried for years is still a thing of beauty. But when it comes down to it, Glock's Tactical Tupperware is without a doubt the best duty handgun I've had! Collect and admire the others. Carry the Glock.
Cheers,
Sirhr