Not much use in early pregnancy but once you are getting bigger and heavier it's brilliant - lean over it to take the weight off your back, bounce around on it watching TV when you are too fidgety to sit still anywhere else, and lean against it to give your back a good stretch.
I'm sitting on mine at my computer, and sometimes watching telly. And a few times kneeling and leaning with my crossed arms under my head on it.
Although the physio's one which is more expensive and thicker rubber feels comfier because it's more squishy - I can sit on her 75cm 'Gymnastic' one fine but had to get rid of my 75cm Reebok one from Argos because it was too big - the 65cm one also needed a lot of air out before it was squishy enough to sit on.
Have around 4-5 weeks to go - tempted to splash out on a better ball (not sure where from) and deflate this one. MrNC and my dad enjoyed playing with it, and Squirmy probably will too in due course...