I've been using AT&T Navigator with my Blackberry because - obviously - there is no turn-by-turn navigation through Google Maps on the Crapberry. That $9.99/month was rough, but it worked well and it was better than buying a separate GPS while I waited to switch to Android.
I have no intention of paying for that now, and as far as I know there's no reason to. Google Maps turn-by-turn is quite good (despite being beta, although we know how Google uses the beta term for years on end) and best of all its free.
AT&T Navigator will probably be one of the first crapware apps I remove from my Atrix.
@SirGeorge: With AT&T navigator you need to have cell service for the maps, and when you first start it up. But it will guide you with basic instructions via GPS even if you lose cell service on your route. Not sure how Google Maps handles that.