@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.