Me.Value will not work in SQL Server Reporting Services 2008 (SSRS 2008) if you have some complex code behind. This is regardless whether you use that code in your expression.
For example, your Tooltip might simply be =Me.Value .
Not only will that not work, but the combination might leave you without a visbile value in the cell. What??
I use quite a bit of code behind the form, and references to assemblies. This did not hinder the usage of the Me.Value property in RS 2005, but it now breaks in 2008. If you need your code behind, the only way around this is to explicity reference the report item.
Example:
To dynamically change the color property, of a cell in a Tablix refrenced as Textbox2...
instead of =IIF(Me.Value<0, "Red", "Black")
try
=IIF(ReportItems!Textbox2.Value<0,"Red","Black")
