I’ve ridden Penmachno a few times. I’d advise the main loop which is approx 19km. The additional bit (another 11km or so) is pretty pedally, without much payback (though it does offer some stunning view of Snowdonia) and is also probably the wettest bit.
Thing to bear in mind is that Penmachno isn’t an out and out trail centre like Llandegla or Cannock or Coed y Brenin. It’s more of a 50/50 in that in places it is more akin to natural, rather than trail centre, riding. As far as I know it was built, and is maintained, primarily by locals, doing it in their own time. As such it doesn’t have the same level of investment (money, time, expertise) as other centres.
That said, what it does have is some absolutely cracking, hardcore singletrack. By that I mean it is full of rocks, drops, ruts and sharp descents. It has bits of it that are true white knuckle riding and as a poster said above the last mile or so is absolutely brilliant.
My two penn’orth is that if you want to ride some brilliant, fast yet hard singletrack that’ll shoot you out of the bottom glad to be alive and glad to be alive, then you could do a hell of a lot worse than Penmachno. The fact that it is a bit wet in places is doesn’t take away from the fact that it has some absolutely brilliant riding that will properly put a grin on your face. The wetness just adds to the challenge in my opinion 😉