iPhone 2.1 Software does install Apps Quicker
I just upgraded to the latest iPhone software and one thing I can say for sure is Apps install so much faster using the new software. They also update much quicker and when the do update they install back to their original position in on your screen. I found it annoying that I had to manually re-arrange apps from the free slot it was updating on to the original screen location after updating the app. It's all good so far. Backups also seem faster.
I have no idea if signal strength, reduced drops, or other phone features have improved yet.