After freezing, they are no good as a raw food but are just fine as part of a mire poix for soups, stews, etc.
I buy whole celery (not the trimmed cut pieces) as it's the least expensive. As a result, I end up with lots left over. I trim off the leaves and little pieces on the top and save them separately for stocks (I save parsley stems, too). The stalks go into a ziplock and are frozen. When I need some celery, I just go in and break off what I need.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan