For a commercial software RPL is even worse than GPL.
It is true that you have to publish your source code on any derivative work.
Also in GNU web site it says:
The Reciprocal Public License is a
non-free license because of three
problems. 1. It puts limits on prices
charged for an initial copy. 2. It
requires notification of the original
developer for publication of a
modified version. 3. It requires
publication of any modified version
that an organization uses, even
privately.
source
Edit
Quatation from RPL 1.5:
6.0 Your Obligations And Grants. In consideration of, and as an express
condition to, the licenses granted to
You under this License You hereby
agree that any Modifications,
Derivative Works, or Required
Components (collectively Extensions)
that You create or to which You
contribute are governed by the terms
of this License including, without
limitation, Section 4. Any Extensions
that You create or to which You
contribute must be Deployed under the
terms of this License or a future
version of this License released under
Section 7. You hereby grant to
Licensor and all third parties a
world-wide, non-exclusive,
royalty-free license under those
intellectual property rights You own
or control to use, reproduce, display,
perform, modify, create derivatives,
sublicense, and distribute Licensed
Software, in any form. Any Extensions
You make and Deploy must have a
distinct title so as to readily tell
any subsequent user or Contributor
that the Extensions are by You. You
must include a copy of this License or
directions on how to obtain a copy
with every copy of the Extensions You
distribute. You agree not to offer or
impose any terms on any Source Code or
executable version of the Licensed
Software, or its Extensions that alter
or restrict the applicable version of
this License or the recipients' rights
hereunder.