I have all my emails duplicated, one sent to my POP3 box and one to my Gmail account. The POP3 box is checked from home using Thunderbird and when I'm at work I use Gmail. The problem with this set up is that it says, twice, that certain emails are unread which in reality they're not. I can live with this for the moment.
Until the other day I had not been using Thunderbird for a long time and when I returned I had 224 unread filtered messages in my inbox. Most of them I had read already on Gmail but several were new. This alerted me to a big flaw in Gmail: it's anti-spam filter.
At home I use one of the best (read efficient) anti-spam filters available, namely SpamBayes. It rarely fails but a spam message slips through about once or twice per week; and I get a lot of emails. It's very rare that a normal email gets trapped as spam but it has happened.
Today I went through (well, some of it) my Gmail spam folder. It has - at the time of writing this - 1005 unread emails in the last 30 days. In there I found lots of emails that aren't spam. For example credit card receipts from Worldpay, server email warnings, emails from friends, email notifications from some of my sites such as the issuetracker and last but not least: News alerts from Google!
I have never trained my inbox to recognize News alerts from Google as spam. Couldn't Gmail be clever enough to look at who sent email and realized that it's not spam?
Gmail is a great free service with a wonderful user interface but the anti-spam filter is far too hungry. Unless you check your spam, do, because you might miss lots of important emails.