Is the navigation user friendly and easy to use? I don't know if I should have it installed, the one provided on the site. Also, if i have it installed, will i be able to have a MP3 player installed along with the navigation system?

The BMW nav system is easy to use. I've had it for 1.5 years. Was a bit tricky to setup destinations at the begining but you get use to it.

MP3, Always on TV, DVD and those type of capabilities you will not get - but there are companies that offer extensions to enable this:
www.nav-tv.com and www.TVonNAV.com

I have not tried it, but you can get the reviews from www.x5world.com

Hope this answers your q.