[bradleyland]

Not a very good option in my experience. Google has some pretty complicated abuse detection algorithms in place. For example, if an account sends email through SMTP, but never receives any email, logs in via POP, or through the web interface, the account may get shut down. Also, Google's tolerance for "Mark as Spam" on the email you send is very, very low. It boils down to this: Google Apps isn't the same as SendGrid, Postmark, etc. Google Apps is intended for use by people, not web apps.

Having said all that, these are my experiences. I've tried it on three separate occasions in the past, and every time, I ended up fighting deliverability problems until I finally moved to a different solution. The most common is that the Google Apps user through which you send email gets disabled for "abuse", and you have to log in through the web interface in order to re-enable it.