I look forward to getting a 4-pack each year, which, BTW, is the only way it is sold here in the US. Each year is marked for the year it was brewed (this year's batch will be dated 2005).
I open two of the four at Christmas time - one for me and one to share with someone special. I save the other two and wait until next year to compare. It's such a strong lager that it can store chilled for a long time and the flavor changes over time. I began doing this in 2004.
This year, that someone special and I will compare the 2005 with a 2004 and my last 2003. I'd get too plastered too soon if I compared them all on my own! I find that the flavor mellows and is warmer over time.
There's some good info on it here