This is a great, albeit rather standard, technique for conditional WHERE clauses. Performance has never been an issue for me as long as the evaluated condition is efficient (i.e. evaluate true conditions wherever possible as stated by another poster).

My only issue with the code as presented is the use of NULL values as the parameter defaults. This is extremely risky since different tools methods of calling a stored procedure can cause different and unexpected values to be passed as parameter values.

Instead of NULL (dates are an exception - see below), use a default value appropriate for the data type. I usually use 0 for number data types and empty string ('') for character data types. Then evaluate the parameter in the WHERE clause as ISNULL(@NumberParm, 0) or ISNULL(@StringParm, '').

For dates, use NULL as the default and then use the ISDATE() function to make sure you get a real date. Since both '' and NULL will cause ISDATE(@DateParm) to return 0, it is safe and robust.