@Yakito in my experience you cannot have a one-size-fits all tools for reporting.  There are two types of reporting needs, AFAIK. One is what I would call forms.  This is where you prints one record at a time, e.g. invoice, quotation, etc.  The other is what I would call analytical reports.

For forms I would advise that you use what Odoo is currently supporting.  As @Stephen has pointed out for v8 it is QWeb.  In v6 - v7 I opt to use webkit, mostly because there was a rumour that OpenERP is going to use webkit starting from v7.

In generating analytical reports, though the Dashboard had provided a great functionality, sometimes the ORM will became a bottleneck if you are processing a lot of data.  This is evident if you use the stock Legal Accounting Reports with a lot of data.  You need a specialized reporting engine that churns out report using a specified SQL.  Most of open source reporting engine out there are Java-based (JasperServer, Pentaho, SpangoBI, BIRT, etc.).  I belive you'll still need to have this kind of reporting engine, be it integrated fully or by way of RPC.  JasperServer [1] [2] and Pethano had their integration modules in OpenERP for you to try.