Is it a good idea or not ?
There is a fierce battle between the big players (amazon, google, salesforce,...) about cloud computing. The components (Like Google spreadsheet) get more and more robust every year.
You could use a Google spreadsheet as a "database" for your application.
If It is for a simple project, It Will certainly work. Google forms for instance uses Google spreadsheet as a backend.
If It is for a critical & more complicated project, only do It if you really Know what you are doing. It Will simplify some things (Like remote access with an excel Like interface) but make some other things harder (Like data backups, replication,...)
The loss of 150.000 gmail accounts 2 weeks ago show that these new models bring new solutions but do not remove all the difficulties)