I've never had a built in satnav and I do like the idea of being without loose cables but I have decided I would never pay extra to have it as standard for the following reasons:
1) The ruinously expensive - ie £150 ish - updates. Most stand alone satnavs will give you this for nothing. Google maps on your phone updates automatically as do other apps like Waze.
2) I have lost count of the number of times I have seen people, in real life and on youtube, using an App on their phone or a stand alone sat nav instead of the built in one. This is proof positive to me that the inbuilt ones are crap.
Having said all that, and contradicting myself, for my purposes, ie very very occasional use, a crap in built one would probably do and would save all that faffing about with sticking the thing on the window and having loose wires dangling about.