Is PhoneGap tool is good for developing mobiles applications? Suppose I am developing iPhone, Android, Windows, Blackberry, etc mobile application development in that case we can use Phonegap or not?
This should be based on your requirements. Cross-platform environments have lots of bugs and issues. Most likely developers used them to create application that does not need a lot of processing and threads and does not need the heavy use of the hardware like an application to browse a web content with a nice views like HTML's.
I use Sencha and PhoneGap before for such application.