Visited 22/01/11: Walked to Disgwylfa Fawr from a parking spot next to Llyn Syfydrin to the west. This is the shortest & simplest route up (my friend's 7 year old daughter had no trouble with it).
I'm embarrassed to say this was my first time up Disgwylfa Fawr and it's hard to imagine why it's taken me so long to make the journey. The cairn is large and earth-covered with an indentation in the top, presumably from excavation. It's a nice lump the stand on and the views of the surrounding peaks are splendid (especially Pumlumon and Dinas). Well worth a visit.
The NMR record (available through Coflein) describes Disgwylfa Fawr and the notorious canoe type things that were found inside it:
Cairn upon the summit of Disgwylfa Fawr, c.20m in diameter & 3.0m high: trenching, in 1937, reported a ring of slab-stones within the body of the mound, within which were found two dugout tree-trunks of oak, 2.4-2.7m & 1.08m long, these subsequently yielded radio-carbon datescentring on 1910BC & 1350BC respectively; within the smaller dugout were cremated human bones, a flint blade & a ceramic vessel, covered by an animal skin.
The author of this text is the venerable Stephen Briggs.