I think DADGAD is a great tuning when used to accompany certain songs and ballads but like anything can become boring, repetitive and overused if the player doesnt think about whatever it is s/he is trying to convey to the listener.

As for tunes there is an excellent book written by Sarah McQuaid "The Irish DADGAD Guitar Book" ISBN 0 946005 93 1 and published by Ossian Publications Co Ltd, PO Box 84, Cork, Ireland. A tape is also available to go with it.

Even if you don't intend to play celtic music, and I don't, it's a good introduction to chord structures etc.