I have used PhoneGap, I liked it. I used AngularJS in conjunction with it.
Everything worked out well with it. Any specific questions let me know.
I would say 95% of the code ends up being cross-platform. There will be a slight bit of branching. I ended up having to use some plugins (native code with functionality exposed via JS), but those were easy enough.
It well and truly becomes an exercise in web-app development. My JS skills were neophyte at best so any issues I ran into were not so much the platform or the PhoneGap API, but just JS development.
On JQM vs Angular. JQM will give you a much better looking app and will have a more "native" feel, but Angular, I felt, allowed you to write a much better structured app. What I ended up doing was ripping the CSS out of JQM for the controls I wanted and just using that with Angular. I do not recommend using both JS frameworks as this will lead to MASSIVE headaches, especially if you are new to either one, as you won't know if the error is due to you writing something wrong or the 2 frameworks fighting.