I have both Navbox and Flightstar. Navbox is perfect for your VFR bimbler who just wants to run a basic what if and PLOG around the UK.
Going further afield Flightstar is the biz. I use the IFR version and its airways planning is superb, never had a flight plan from it rejected. The 3d profile and the airspace markings are excellant and allow for very accurate long distance planning. Also being able to pump out the PLOG, W&B and FLightplan are great. I can produce the flight plan and fax direct from the PC all from Flightstar.