It might be something you'll just have to monitor - the thing I like about mifi's though is that at least you can easily monitor data usage. All I can say is that between 6pm (ish) yesterday and 1.30 am (ish) today I used a total of 323MB (that's on my landline ISP) That was a bit of the internet, some emails, a bit of WOW (waiting for my email!) and then about 5 hours of TOR. Oddly my midnight to 1.30 am usage (which doesn't come out of my allowance) was 63MB which means I used as much in 1.5 hours as you did in your full session. But I'm still convinced when I used it on the beta weekend it hardly made a dint in the allowance on my mifi and on my mifi I only have a 5GB allowance (mainly for weekend use away in a caravan TBH)

Yep I played one of the beta weekends on my mifi - I suppose it depends on the network etc but where I use my mifi I have an excellent 3 connection. Pings were in the mid 100ms and there was 1 or 2 spikes a day which could have been the network or the game servers. So totally playable and will be doing so again. Data wise the hit was barely noticeable (and this was a full Friday to Sunday) so again no concerns at all.