PhoneGap is an HTML5 app platform that allows you to author native applications with web technologies and get access to APIs and app stores. It is open source and supports many native features over 7 mobile platforms.
After trying many featured apps built using PhoneGap, I see that apps quality are quite good but some times screen flickering happens (which gives you the feel what you are using a browser).
Some stackoverflow posts shows that these flickers can be solved but I did not give it a try.
Another thing I noticed is that a lot of PhoneGap apps are available for Android and iPhone only and even some of them are available just for iPhone which means that PhoneGap targets web developers who wants to make a native mobile version of their web app quickly to reach iPhone and Android users.
For native mobile developers I believe (not only me) it is better to use native SDK if they are much concerned with quality and don't want to learn web technologies.