Camped at Dyffryn last bank holiday week, all I'd say is keep an eye on your kit, came back from a cracking buggy session on the beach to find some little scrote had thrown a lump of concrete through the roof of my tent, also if it's busy the showers can be a real pain with the cards not working.
other than that access to the beach is great and with an on shore breeze you can buggy all the way up to shell, but it's a long walk of shame if you get caught out by the wind.
beach at Dyffryn was nice and hard, shell seems to change on a tidal bassis but there's always a spot to buggy, but you do have to walk along the beach to the designated area at Shell, where as Dyffryn, you can just find yourself a quiet space and get on with it.