My girlfriend and I just got back (yesterday) from a little vacation in Cabarete (spent a few nights at the Viva Tangerine). The drive from S.D. is more or less 150 miles and takes about 3 - 3.5 hours depending on the driver... I have a problem going less than 80-90mph though...
If you're driving yourself (and arent used to D.R. driving), be careful on the road from Puerto Plata to Cabarete. Going through the mountains before Puerto Plata is "interesting" but nothing to stress about...