It is excellent, I have had the BMW one, the Mercedes Comand, the CD based RNS-D from Audi, units from Navman and the latest TomTom and had the new DVD RNS-E from Audi in the last 3 cars, and have to say that out of all of them I prefer the latest Audi units the most.
If you could add your own Point Of Interests it would be perfect, but that is the only thing that is not as good as say the TomTom, evrything else I find alot better, multiple routes on start up, instructions in the DIS and the ability to be able to play all your music off SD cards with artist and title displayed on screen.
I was offered an extra £1000 on my 3 series touring when they realised it had the full navigation, Mercedes paid me £1200 more for the ML because it had Nav and phone and when I was considering a trade in on my A4 avant he said that anything with Nav sells straight away, which is true, I have sold 6 or 7 cars over the last 2 years and all have had Nav, and all have sold within a couple of days, it makes them stand out from the rest.
Also with alot of new cars now adding Nav as standard it won't be too long, couple of years at most, before a car without nav will look underspecced.
It has gone from being a £3k option that you may get £500 back on, but help sell your car quickly, to a £1500-2000 option that will return you £1k. If you do the sensible thing like you are considering and get someone like Craig to fit it for you for a grand then you can't really go wrong, you can enjoy it now and get your money back come resale time, and sell the car far faster.
You will be surprised just how much you actually use it, even on local trips it comes in handy, well worth having.