Is Gmail filtering spam properly?

When Gmail first launch, users were surprised to see their almost spam-free account. As time goes by, spammers actively dig for more Gmail accounts to spam but Gmail's advanced filter blocks almost every single spam which saves users' time even screening through the subject of the spam to see if mail are wrongly marked as spam. However, don't know starting from when, spam appears in my Inbox (in a large amount) and mail are wrongly marked as spam. Now, I have to frequently check my Spam box to see if any important mail is wrongly marked as spam which of course takes me a lot of time. Another problem is that there is no 'Undo' for deleting spam permanently. At least like archiving mail, an Undo link should be provided. I don't know if I am the only one experiencing the problem or are spammers able to go through Gmail filters. Hope blogging about this can raise attention and hopefully Gmail engineers will address the problem soon as everyone knows how annoying spam are.

Note: I already mark every spam as spam to teach Gmail what to filter.