Hi Srp,
Masa, along with Atlas are one of the biggest agents in Spain.
I have no experience of Masa, we bought via Atlas.
Justin the site owner, did a survey not long ago, which compared Agents & Builders in Spain.
This was a survey answered by EOS members.
It might help you to decide if you have a look at the results.
I imagine the link to it is on EOS Homepage.
If you can't find it, PM Justin and he will point you to it.