Pentaho Reporting and BIRT are very close in feature and functionality. Here are some key differentiators in functionality.
1. All properties/attributes on an object or report section can be conditionally applied using an Excel like formula (Open Formula). Thus, there is a easy and standard method to conditionally set an objects or report sections formatting, background color, size & position, visibility, etc.
2. Report against just about any data source; jdbc, mondrian olap, pentaho metadata, xml, scriptable (java script, groovy, etc.), static table, or hibernate.
3. Report against multiple disparate data sources (relational, csv, text file, excel, xml, web service, etc) as one data set using a Pentaho Data Integration transformation.
5. Easily create reports with parameters/prompts and publish them to the BI server where users can view them via the web based client, the Pentaho User Console.
I also recommend looking at the sample reports in the Report Designer which will give you a very good idea of what can be done in Pentaho Reporting 3.5.
I've also included some screencam demos links of the product below.