We are going to Germany for a few weeks this summer taking our adult boys to see my wife's relatives and sightseeing. Unfortunately, relatives are in Essen and Fussen on opposite ends of country. We are flying in to Frankfurt & plan to rent a car and go to Essen first. We are hoping to then go to Paris for a few days and then to see relatives near Fussen. Is it feasible to drive from Essen to Paris and back to Germany or should we drop off car, take rail to Paris and back to Germany and then rent another car?
Would do the second option. Paris can be a nightmare by car, and the Metro will get you anywhere you need to go in the city.
Normally for tourists, a car isn't a necessity in Germany. But because you're visiting relatives who may live in locations not convenient to public transportation, I would rent the car.
The way I decide driving vs. rail on each trip is to look at Michelin (or google) maps online, figure out how long driving will take and how much it will cost to have the car, vs. how long long the train will take and cost. Since you include Paris don't forget to factor in the cost of parking there, and the hassle of driving there. I think you really have to do the legwork to figure out which is the better option.
A car would definitely be a burden, not an asset, in Paris. Take the train from Essen to Paris and from Paris to Fuessen.
If you book far enough in advance (up to 92 days allowed) on the German Rail site, you can get a Europa-Spezial Frankreich Savings fare as low as 39 EUR for both routes.