I'll give this a shot, but this is just one person's opinion. Take it as such.
My wife and I did not rent a car. The biggest negative was being captive to the resort's restaurants. Even if you find the meals okay (they were decent), for a full week, it gets tiresome. There is no where else in Gamboa to eat. (Not quite true. We did purchase one meal from a food stand in Gamboa.)
As far as driving, for the most part, it would be equivalent to what you saw in Costa Rica. (Maybe better in that the roads are marked better.) The one very notable exception is that there is no way that I would be willing to drive in Panama City. (While I did not have a car in San Jose, CR, I think I could have managed that city if I really needed to. From some of what I saw in Panama City -- eeek!)
We had the resort arrange for a guide to show us around Panama City. They found a very nice college student who did an excellent job of showing us around (and, as far as I am concerned, saving my life by doing the driving).
If you do make it to Panama City, ask about the craft market at what used to be the YMCA. (This was the description I used and it worked.) That was the best place to find crafts (classy crafts) to take home as souveniers.
If you are an avid nature buff, bird lover, whatever, I really recommend making arrangements to take a tour of Barro Colorado. The entire island is a research center for the Smithsonian Rainforest Research Institute. A tour has to be arranged with them in advance of your trip. Your point of departure will be dockside in Gamboa and you can walk there from the resort.
There is a trail on the far side of Gamboa which is often mentioned as one of the best places in all of Panama for sighting unusual tropical birds. (The name escapes me at the moment.) It is bit too far to walk to the trailhead from the hotel even for avid walkers. You can go there as a paid activity arranged with the resort. The guide we had was an excellent bird spotter-identifier. Fortunately, we were good enough (no matter how good you are, it helps to have an extra set of eyes), he was willing to take us along for free on future trips.
Finally, it you want to read one thing in advance of the trip, something that will help you understand life in the rainforest, I would recommend "Tropical Nature" by Adrian Forsyth and Ken Miyata. It is not at all technical, very informative, and, at times, just fun to read.