We've received a few notices from Google regarding a $20 Million class action settlement.
In reading over the legal document, it really only applies to Google Adwords accounts using daily budgets to cap spend. Furthermore, it pertains to instances where Google has overcharged advertisers an amount beyond that Account Budget Overrun. I believe this has previously been billed to advertisers as "Account Budget Overrun", the total amount charged above and beyond the daily budget limit. Within the document, Google denies any wrongdoing.
Don't take this as legal advice, but...
If you have used Google's campaign daily budget feature over the past few years and have frequently been "overcharged" (above and beyond your daily budget), it might be worth taking a look at the settlement. Otherwise, it may not be worth investigating or pursing.