To offer a singular data point, contrary to other posters here, I am not seeing this at all, and I pretty actively use my two Gmail addresses which have been active since 2005. My spam inbox regularly gets correctly categorized spam, and important emails still correctly land in my inbox.
It is incredibly strange to hear from you (and others in this thread) who don't have a spam problem with GMail. From everyone I've talked to, it seemed endemic. From my experiences with my own account (circa 2004), GMail has moved backwards in recent years. I'd say it became a serious problem a few years ago. Marking things as spam is useless because then you immediately start getting legitimate mail marked as spam.…
The number of spam messages I get in general, judging from what shows up in the Spam folder, varies wildly from week to week. Sometimes I'll get five over a two-week period, and then get a week where I receive about 15 per day.