DAX Function Guide

ISEMPTY returns True if the table is empty (doesn’t have any rows), otherwise, False.

Syntax using ISBLANK is more readable, and it could also produce some benefit in the query plan. However, when the table is not empty, you are always counting all the rows of the table, even if this is not strictly required. Microsoft added a DAX function in SQL Server 2012 SP1 Cumulative Update 4, so any build of Analysis Services 2012 and Power Pivot for Excel 2010 higher than 11.00.3368 can use the ISEMPTY syntax. It is simpler, meaningful and also faster. If you use DAXMD (query DAX on a Multidimensional model) the performance improvement offered by ISEMPTY is very important, and you should avoid using any COUNTROWS version to check if a table is empty.

Unfortunately, ISEMPTY is not available in Excel 2013, so using it in an Excel 2010 data model in Power Pivot will break the calculation once you upgrade the workbook to Excel 2013.