I've had the iPhone 2G, 3GS, and now the iPhone 4. It's the only Apple device I own, aside from an Apple Cinema Display on my PC.
I am able to fully reproduce the signal issueto the point where I can totally stop data transmission and miss a call. In real world use, and being left handed, I haven't actually dropped a call or been unable to browse a website.
It's quite easy to reproduce the problem, even in areas with 5 bars. I actually live in an area where the signal is quite low, and even in my house, I am not dropping anymore calls than I have with the 3GS.
I recommend the iPhone 4 with caution. The caution is only because, depending how you hold the phone when browsing the web or when in call, you may or may not run into an issue...and then again, you may not even be in an area where you are able to reproduce the issue. I personally cannot reproduce the issue EVERYWHERE...only in certain places.
Otherwise, the iPhone 4 is an amazing device and despite there being obvious hardware flaws, I simply do not wish to go back to the 3GS since the design flaws in the iPhone 4 don't effect me in the real world.
YMMV