I used one today. I had no problems with the ticket.
National Rail says:
However,
Transport for Wales don't list anything like that. I've left a message on the Ffestiniog WWW site to ask for their comments.
I live in Ely, and to do it as a day trip meant an early start and a drive. I didn't want to buy a ticket in advance in case I overslept. In the end I set off at 4am and got to Sandwell & Dudley by 6am.
Sandwell & Dudley, has a big free park, and is close to the M5. The ticket office opens at 5.20am. I had thought of starting from Smethwick Galton Bridge, but its ticket office only opens at 6.15am, just after 2 trains have left towards Crewe. If you get the ticket beforehand, then it might be better as you are less likely to need to change at Wolverhampton.
The ticket was printed on paper with a 2D bar code, which worked fine for all the ticket checks, but not at the barriers at Shrewsbury and Wolverhampton. None of the staff had heard of it. It took a few minutes at the ticket office to find out how to print it, with codes at
http://www.railrover.org/pages/ffestiniog-round-robin.html
I did the journey anti-clockwise with the 12:25 train from Blaenau Ffestiniog to Porthmadog, and then stopped off for an hour in Shrewsbury on the way back.
I got caught out at Chester. I wanted the 7.58 to Llandudno which was shown as being on platform 4B. I ended up on the 7.50 from Llandudno towards Manchester which also went from platfom 4. 2 other people also got caught out by this.
Today the 18.06 from Porthmadog to Machynlleth was cancelled due to "resource availablility". It said that there would be road replacement transport. I don't know if that would have been fast enough to get the connection towards Birmingham.