Are you talking about an external GPS or do you mean the Point-1 compass? The compass is a must in my opinion. This is not because of the overlay, but to keep the boat pointing in the right direction on the chart while you are sitting stationary. If you use GPS to determine your heading, it would spin around and around when you are in one spot. The compass makes your boat point in the correct direction on the charts at all times.
If you have an autopilot, you must get the Point 1AP model instead of a standard Point 1 compass. The Point 1AP does not filter out data so its more responsive on an autopilot system. The standard Point 1 compass filters data and only sends in the data with a lag. That lag is no good when you have an autopilot moving at slow speeds. It would make S-turns since it does not get data fast enough to make quicker adjustments.
Get the Point 1. Radar over chart is a great feature, I use it to determine if a spot is occupied and, if so, how the boats are positioned. This help me decide if I can effectively fish a spot or not, SOP at places like Izor's and Catalina.