It is doing a decent job actually filtering, and it is quite transparent in that it explains why it decided to filter things. Sadly, my email is a real challenge to filter automatically.
I get exactly the kinds of emails that often look like spam to automated systems. They come from random addresses including aol and hotmail and home ISPs that often don't have a great spam reputation, they often contain HTML and embedded images. The Oldies Club ones contain sad stories and pleas for help (admittedly the real ones don't then go 'and I put $500,000 in a Bank of Nigeria account...' but you can see how the confusion arises. ) Others contain prices and instructions to add free offers or special deals to various websites.
I often get emails from people who have not emailled me before, or from people using more than one mobile email address, and that's an issue because one good way spam filters work is that they build a list of approved non-spam senders. For most people that works pretty well, as most emails come from people who have emailled before, but it's not so good for me.
One thing that did get spam filtered was an email from philmophlegm's car insurers, which was quite phenomenally badly structured, and looked terribly spam-like. If it hadn't been that a) I was checking the filter because it is new and b) I knew it was his renewal date, I'd never have sent it on to him. As it was I strongly suggested he use the phone number from the website, not the one from the email, to check if the email was real. And it was! I was ever so surprised.
I just deleted all the spams sent to me on 18th March. There were 759 of them. Lucky I don't have to read the buggers.