SPAM filter onboard Gmail is pretty good. Actually, I haven't got any problems with it (the same amount of false positives as in all other similar tools) up until now.
I receive a daily backup summary generated by my own server. Backup process runs every day at 3 p.m. and about half an hour later I get a summary e-mail. Since this is auto-generated message, its content is always nearly identical. Some filenames differs sometimes . It's been working for over a year. And for over a year Gmail has been accepting this e-mail without any problems and it always landed in my Inbox. Today, for the first time, I found it in my SPAM folder. Why? Gmail informed me that its content is very similar to SPAM messages.
Quite interesting. For over a year this e-mail hasn't been suspicious to Gmail at all. Of course, I have never marked such messages as SPAM manually. And all of sudden, after receiving about four hundred of them, it started to wonder if it might be SPAM.