We have had a number of threads discussing this and there are many good key holders but there are few dodgy ones.
Example one I started
www.eyeonspain.com/forums/posts-long-21220.aspx
I would suggest that any key holder you appoint ensure they are a fully legal business (company or autonomo etc.) and registered (recorded) with your home insurance provider and bank if you have a mortgage.
In addition it may be worth getting a contract / agreement in writing that clearly defines responsibilities, payment terms etc.
It may cost a bit more for a legal company rather than using a “neighbour” but this is a valuable asset we are talking about and why take unnecessary risks to save few euros.