Have had these "Sapphire Skies" yucca rostrata since July 3/08.
When they arrived:

Planted them all into the tall plastic Slushy containers, big drain hole punched into the bottom.

Fall of '08, planted two into 2 gallon plastic, three into a communal clay pot, and one stayed in the tall plastic Slushy.
Soil in all is poor, seldom watered, hardly ever fertilized.

Today's results:

So I believe giving them lots of root-room equals a thicker yucca. Communal clay pots = no good.
The original plastic Slushy container = taller yucca, but with more die-back of leaves at base.

Wish I had been able to afford these (from a website):

Next year I'll do some fertilizer tests on them.
Barb