So far, I have downloaded several applications, mostly which I don't really need, to test the experience of PhoneGap based application. I should say, given the limitation, its quite impressive. However, if you are being really picky, its easy to notice if you have say, a list on the application, scrolling the list after the view is opened (even if its the second time), the scrolling isn't so smooth and stutter a bit.
Of course, for normal people (aka non-developers), they may not notice, much else know if it is a PhoneGap application or not.