I was using the iPhone 3G for about 2 years, and jumped to the HTC Desire when it was launched a few months back.
People are talking about how fast or "smooth" iPhone is, but i think my HTC is better (compared to 3G at least). Plus, everything on the iPhone is so "restricted", e.g. ringtones, wallpaper (i think OS4 has solved this), icons based interface, etc are all fixed and you cannot do anything except jailbreaking it.
With HTC, I can choose to have widgets instead of icons and vice versa, themes, wallpaper, ringtones, etc. The customisation is endless. I'm enjoying my phone and i never regretted jumping