I pre-ordered and received my iPhone4 switching from my Motorola Droid X. There is no comparison. The Droid X is too large. The battery life is average at best. The screen flickers and is not fluid switching between applications and the home menu. Some of the apps are untested and unrefined. The iPhone4 build quality is top notch, the screen resolution is awesome and the apps are refined and polished. When you switch between screens and the home screen and apps, it is very fluid and smooth unlike the Droid X. The Battery life has been great, just charged it today since receiving it 2/7 and still had 50% battery life remaining. My Droid X is in the closet and I'll not go back.
I would suggest the Droid X all day. Larger screen (no it's not too large, there's a reason 4.3 inches is becoming the standard on all top end phones), more free apps (75% compared to 23%), voice to text, free navigation, widgets, etc. The iphone to me just feels extremely limiting when coming from a quality droid like the X.
the droid does more in stock form. loved it, but it does have the none smooth feel sometimes. the Iphone4 is SMOOTH everything works. BUT you are very limited on what you can do. Jailbroke it the first day. then its on par with a droid, and smoother, prettyer and has all th other iphone good ness.