I'm not fully equipped to give you the technical details, but I will offer my experiences...
When I got my Droid, I didn't even bother opening the OEM charger, I used an old charger from a LG Nextel phone I had laying around. It worked for awhile, but one day, it suddenly just, stopped. It won't charge my phone at all, even though the charger still in fact works, since I'm using it to charge my bluetooth headset as I post this.
I decided to finally break out the supplied power brick + USB cable, and the phone charges fine, although, it seems my battery life has taken a slight hit. I'll probably by another one soon. As far as charging via USB; it works just fine. but you're not getting as much juice as you normally get from an outlet. I haven't experienced this on my Droid, because I have yet to try it, but in the past, I have had my phone connected to my PC while on a call, with WiFi going, and I'd get a message telling me that the current power source isn't strong enough to sustain a charge. That's not the exact message, but that was the gist of it.