Tried out a dual-layer disc the other day. Tried it on

1) PC with a year old DVD burner - played but was very stuttery.
2) Oldish Sony player (can't be befuddled to check which one) - worked perfectly
3) Xbox. Refused to play.
4) Another PC drive - refused to play.

Will experiment more with more details soon. However, I ended up re-encoding the video at a lower bitrate to send to the client on a single-layer DVD. No visible difference in quality anyway!