Dub -
I was in South Wales this spring for a couple days of golf. Played Pennard, Tenby and Ashburnham (couldn't get on Porthcawl because of British Amateur).
I would consider Pennard a "must play" in South Wales. It is tremendously fun golf in a beautiful setting, complete with castle ruins, cliffs, cows, and wild ponies (for details on Pennard, check out the course profile on this site).
If you are driving through Tenby on your way South to Porthcawl, you should certainly stop for a game. Not quite as much fun as Pennard, but still classic links golf through some interesting terrain. While three of the final four holes are routed through parkland and left me with a bit of a bad taste in my mouth, the terrific first 14 holes more than make up for it.
I would play Ashburnham only as a filler. Apparently, lots of people think highly of the course, but I was disappointed. The first 8 and last 3 holes are basically parkland and (I thought) quite boring. Holes nine through 15, which are routed closer to the sea, were a lot of fun, but I don't think six holes are enough to warrant a side trip to Ashburnham.
Have fun!
-Dave Q