I've had a lot of good experiences with TomTom in the past, and they keep the app up to date with their new map data. I didn't like MotionX at the time since it was very slow to reroute if you missed a turn, didn't have proper avoidances, and wanted me to do things like take the HOV lane even though I was the only person in the car (this then caused the app to freak out and try to route me through emergency vehicle only areas to get back on the HOV).
TomTom is also stores stuff locally without needing to cache data, and has addons like RealTime traffic, et al. I think TomTom and Navigon are the go-to's for a full local-store GPS app. MotionX might be viable if you want to save some cash and only use GPS from time to time.