I recently flew on an Emirates A380, which comes with inflight wifi for the entire 7 or 8 hour trip. The cost is $20 for 100MB (about £12). Before anyone freaks out at the "cost", consider that there are hotels in the UK alone that charge £5 for an HOUR of net use (how does that even happen, anyway?)
Don't even get me started on those standard "inflight email services" where it costs a small fortune to send a solitary email.
The connection was fast and from what I can recall, only went down once and quickly reconnected. They do give you a warning that there may be some countries you fly over where wifi isn't available, but considering we were flying over Iraq with a perfectly fine signal I can't think where this would actually flatline.
Having said all of that, 100MB over 7 or 8 hours could still be a stretch for some people, so:
1) Make use of the (realtime) monitor that shows you how much you've used.
2) Skype generated a steady stream of megabytes being used up simply by being logged in...didn't matter if I was talking to anybody or not. MSN messenger didn't do this, and even when chatting barely registered on the monitor.
3) Switch off images if you don't need them, especially if you're hammering Twitter.
By the end of the 8 hour flight, I think I'd gone through about 70 to 75MB. In fact, I ended up switching images back on so I may have been overcautious. Still, worked great and I'd definitely use it again. If only more airlines had this service...