Pentaho seems to be pretty solid, offering the whole suite of BI tools, with improved integration reportedly on the way. But...the chances are that companies wanting to go the open source route for their BI solution are also most likely to end up using open source database technology...and in that sense "database agnostic" can easily be a double-edged sword. For instance, you can develop a cube in Microsoft's Analysis Services in the comfortable knowledge that whatver MDX/XMLA your cube sends to the database will be intrepeted consistently, holding very little in the way of nasty surprises.
Compare that to the Pentaho stack, which will typically end interacting with Postgresql or Mysql. I can't vouch for how Postgresql performs in the OLAP realm, but I do know from experience that Mysql - for all its undoubted strengths - has "issues" with the types of SQL that typically crops up all over the place in an OLAP solution (you can't get far in a cube without using GROUP BY
or COUNT DISTINCT
). So part of what you save in licence costs will almost certainly be used to solve issues arising from the fact the Pentaho doesn't always know which database it is talking to - robbing Peter to (at least partially) pay Paul, so to speak.