Hibernate version: 3

Are there any known reason not to use hibernate's framework to obtain the JDBC Connection?

The issue is:
We have a large scale web app and are slowly migrating parts of it to start using Hibernate. There is a lot of custom SQL written and we already have a pooling solution in place.

Is it bad practice to do:

Session s = SessionFactory.getSession();
Connection c = s.Connection();
PreparedStatement ps = c.prepareStatement(CustomSQL);
ps.execute;
s.close();

In my research I plan on doing some tests to see the times it takes to execute some queries through the 2 different methods. I'll post those results here. I was just wondering if someone out there knew better already.

Thanks in advance.
- Jonathan