I have the adapter, 20 v to 18 v only. My adapter is very difficult to remove from a tool. I think I'd like to have an adapter for each tool, but I have 3 drills, the circ saw, sawsall and jig saw. That's five adapters. My hands aren't as strong as they used to be, so maybe the stiff release buttons will be easier for others. I can use a pair of giant channel locks to press in the release tabs, If that's the norm, I hope DeWalt will reengineer that release, it's the only fly in their ointment.
The new batteries are really expensive, but they run for a long time before discharging. You also have to buy a different charger. You pull the 20 v off the adapter to charge it (easy) and you can't charge it in the old charger. At least DeWalt isn't leaving you out in the cold. You can buy off-brand batteries in both sizes, but they don't last all that long and have a high failure rate. I really love that circ saw and the jig saw. Not wrestling with a power cord is nice, and DeWalt has a nice selection of blades for the circ saw. Use Bosch blades for the jig saw, much better results.
The more I do, the less I accomplish.