True this isn't the most interesting site but I believe that there were large stones here once as the surviving stones of similar material to other local standing stones and the pieces also appear to have been broken relatively recently, many of the shards have flat sides.
There is also large recumbent stone in the hedge of the field (to the East) I'll post a picture soon.
Although this site alone might not warrant a special trip the surrounding area is quite spectacular, with a most impressive Hill Fort - Caer Cradoc clearly visible across the valley.
Visited 2nd January 2003: Pen y Wern sits at the top of the hill (the one it takes it's name from), at the top of a cultivated field. The area immediately around the site has been left unploughed and there are a couple of sorry looking Scots Pines standing on the south west edge of the site. The view across the valley towards the small town of Knighton (aka Tref-y-Clawdd) is presumably quite impressive on a good day, but unfortunately it wasn't a good day when we visited. Unless you enjoy climbing hills, I'd recommend approaching from the north because it's a relatively gentle slope up to the site itself.
I'm afraid that there's not much to see at Pen y Wern, and I'm pretty convinced it's a cairn rather than a stone circle. There are some stones on the ground, but they are relatively small. I'd hesitate to suggest that they are anything but cairn material, but I may be wrong. The edges of the cairn are difficult to discern, but the stones that can be seen don't appear to correspond with the contours of the site, so probably not a kerb cairn. There is a large hollow within the site that may relate to an illicit excavation at some time.
Unless you're already in the area I wouldn't recommend making a trip just to see this site.
English Heritage records state ring-cairn rather than circle:
The monument includes a ring cairn situated on the rounded summit of Pen-y-wern Hill. The ring cairn survives as a flat-topped circular mound 30m in diameter and up to 0.9m high. The mound is irregular and hummocky over much of its upper surface and a shallow hollow 5m in diameter and 0.3m deep lies south west of its centre. Around the perimeter of the mound are eleven earthfast kerb stones, the largest with dimensions of 0.8m by 0.4m. Other stones scattered across the surface of the mound are loose and have probably been disturbed from the cairn edge. Although no longer visible as a surface feature a ditch will surround the mound with an estimated width of 2m. An associated stone 150m SE, formally standing upright, is related to the ring cairn, though the subject of a separate scheduling.
A stone circle of uncertain status - perhaps just a cairn but it is 27m in diameter - with an outlying stone(s). Pen y Wern is the name of the hill. Pen means top or head, the wern part means 'alder swamp' apparently (a stream does run through the valley below).