I have a standard alfa connect nav, not blue and me, and I have to say it is one of the best things about the car.
Sure, it has its faults - it takes ages to get the address in, and you cant use postcodes. But once you get over that, it works fantastically. I found it frustrating to start with, but once you get used to it it is really very good.
I use it not only for finding addresses, but also as a permanent map. I keep it zoomed out to a decent range, always facing north. It is like having a road atlas open on the passenger seat, but you don't need to keep turning the page. It gives you sense of where you are, what is a round you. If used like this, it takes away the "brainless" mentality of following a sat nav, which is the norm for many people nowadays.
It also means you can easily see alternative routes, work out ways round traffic jams, etc.
Compared to portable sat navs, it is a different world. It doesnt require you to plug it in, stick it on the windscreen and set up before you leave, put it in your glovebox after, then worry about when it will get nicked. No wires everywhere and it doesnt keep falling off the windscreen. And you can use the drinks holder for drinks!
The built in phone is not as good as bluetooth because you have to occasionally updaste your address book and you also need to pay an extra few quid a month for the sim card, but it always works, is easy to use. If I was starting again I would add a parrot kit and not bother with the extra sim. On the plus side, it means your car always has a phone, so if someone else uses it they do not need to pair their phone.
I'm sure that there are better after market systems, but this is definately worth having and a million miles better than a portable sat nav.