I asked a lot of questions about driving (renting a car vs taxi) so I thought I'd share my recent experience.
The drive from SDQ to Samana area wasn't nearly as bad as some made it out to be. Just follow the Quality Inn signs to make sure you get off the highway at the correct spot to make the tollroad. Return trip a little tricky to get back going east on Las Americas but manageable.
Drove around in Las Terrenas town, a little nutty w/ motos/dogs/people but just keep calm and keep your senses about you. Also drove out to El Valle for ziplining. This one was a little tricky with directions but only 1 wrong turn that was recovered quickly.
All in all, if you want to drive there is no problem. If you are a timid, get flustered type of driver, then you shouldn't drive here. Just need to be a calm, forceful driver and you will be fine.
Total costs for 1 week: $250 for compact rental (includes $9 day liability coverage, CDW carried through my Amex). $50 tolls, $50 gas (400km of driving).
My first time to DR and I speak no spanish. Most on this board will say don't drive, just thought I'd give another perspective. If you think you can do it, you probably can. If you are unsure of your driving, you probably shouldn't. Beautiful country and glad I got to see more of it with the use of a rental.