In 2009 and 32x 2010 I could use a plan regions all day long.
The project we are working on has 5 floors, that are really made up of 9 floors at different floor heights due to additions constructed over the years (3rd and 4th Floor have 3 different floor heights)
To show the duct work and equipment correctly, duct-work and associated equipment is hosted to the corresponding level, and then we use plan regions (about 4-6 for the 3rd and 4th floor) so that these items show or do not show.
There were no issues with this with RME 2009 or with 32x 2010. When we went to 64x 2010 and update 2 we noticed that the plan regions were not working anymore.
I would really like to get my hands on a copy of Update 1 for 64x RME 2010, to see if they really fixed this.