I have. It works but o2 are hot on checking what you are using your sim in. Whilst I didn't use my contract sim, I used a pay and go sim with the iPhone allowance and I got a text telling me I'd lose the allowance if I wasn't using the sim in an iPhone.
I use a vodafone sim in mine now, one of the ones they did where you paid £15 for 1GB of data that didn't expire. My browsing outside the house is rare so this was the best value for money for me without resorting to swapping sim cards all the time.
Just got back from lunch in a cafe with particularly bad O2 signal and the MiFi serviced my iPad and two iPhones at the same time just as well as if I'd have been at home using my wifi.