Never use them is my advice. You are in a world of hurt if you ever have to change the definition. Perhaps this has improved since SQL Server 2000 and someone with more familiarity with the newer versions can tell you whether it is now safe to get in the water, but until I had confirmation of this and had checked it out myself with a test, I wouldn't put it on my production system.
Check out this question for details:
How to change the base type of a UDT in Sql Server 2005?
