Damn. I know it's already been said, but this really was unexpected. I had not heard of her illness, and she still looked good as recently as last season of SJA. And at 63, she could have potentially lived quite a while longer. Really not expecting this at all.
Although I was first introduced to Sarah Jane through School Reunion, I have over the course of my viewings of classic Who seen many of her stories with Tom Baker and even a few with Pertwee. I've never really watched SJA aside from the episodes David Tennant and Matt Smith were in, but it did seem a decent enough show.
Sarah Jane is perhaps the closest thing we have to a companion to multiple Doctors. Proper companion of the Third and Fourth Doctors, guest appearance with the Fifth, recurring appearances with the Tenth, and an appearance with the Eleventh. I was hoping she'd have a few more appearances with the Eleventh, even if it was just something like another team-up with a bunch of former companions ala The Stolen Earth/Journey's End, but sadly that's out of the question now.