I like how they are always willing to try new things, even if it's not something we're currently doing! They were able to adapt quickly when our agency went out of business recently so there was no interruption in service or work being done for us during their transition period (which is very important!).
We've worked with them since 2009 but haven't really found anything specific yet - just general improvements each year as far as SEO goes along with some other areas such as website development/design etc. Our biggest challenge has been finding someone who can take direction well enough without micromanaging everything down too much which isn't what you want from an outside vendor.