Really easy. I fitted a Garmin 66w with no previous experience. The cable fits nicely around the top of the windscreen down to the glovebox where the fusebox is. I used the internal light fuse connection for permanently on power and the cigarette lighter fuse for the ignition switched power. I picked these for their minimal impact if something goes wrong on those circuits.
I think it took me less than an hour to do the whole job and no part caused me complications. Garmin's power connector is nicely packaged in a single cable so no issues hiding the cable for me. Definitely worth doing as looks a lot lot smarter than using the cigarette lighter.
I found the hardest bit was routing cable from rear camera to front one. I ended up taping one end to a long stick and pushing (carefully) between headlining and roof. I pulled rubber door seal down to make sure I didn't damage the side air bags. Other than that it was straight forward.