I'm not sure what your goals are when you mention "usability" -- but I do some work through agencies, and for their clients who don't currently run any good analytics, they do recommend GA as an analytics package option. There is a lot to like, even beyond the "free" part.
I recently saw a report about the GA market share, and even among the top tier websites the number was surprisingly large - I can't locate that article right now but it was something well over 50% for top sites.
The GA reports just seem easier for middle and top management to configure and use. It's also rare for any company to use the full power available in any analytics package. They really need a dedicated analytics person (or even a team) to do the job well - coordinating goals across the many departments. Many companies do not yet appreciate the value in that.
The usability goals I had in mind involved funtionality and content testing / measurement in order to configure page designs, widgets and content for satisfaction relevance. I wasn't too specific in my mind as to whether it was an e-commerce or alternative marketing goal.
I wasn't sure how up to the task GA was versus a package like Omniture or others.
Three really useful tools when it comes to usability are google website optimizer (or is it Google conversion optimizer? There are so many tools now), which does a:b testing of on page content and "Crazyegg" which maps the mouse movements of users over the page. The third tool would be ClickTracks, which looks at where users click on a page in a visually intuitive way.