It's unlikely to be cheap.
All the telcos are now rushing to work out what it's going to cost them and make their prices reflect that. There's no way EstoniaTel are going to be able to pay to use O2 or Three data roaming without paying for it. They will, at best, be charged the same as everyone else.
The EU says they have to be able to roam. It doesn't say that the cost of that won't be reflective of the business costs. And you can bet your backside that all the telecoms companies are now putting out their pricing for their partners to use their SIM on their networks, the cost of which will just end up with the user.
But hopefully, it will once and for all prove that local Internet data on a local 3G SIM/handset costs no more than local Internet data on a roaming 3G SIM/handset. All they do is strip off the data and push it over their leased-line to some peer point in the country, whether you're from Britain or Russia.
What this does mean, though? EU International call revenue is about to plummet. Why bother to call from a phone if you have Skype, WhatsApp etc. throughout the EU for free on your already-purchased contract?
My girlfriend is Italian. We went to France and Italy over the summer. Throughout France, bugger all. Literally no 3G and dodgy GSM connection when we did get it. The locals had NO problems, though. Throughout Italy, better but still no data. But we never once made an actual phone call. What we did was Wifi and use that to call, or we just did chat online. We had an Italian phone/SIM, same kinds of problems in the UK / France, except when it was in Italy itself. But we still never really used it for calls.
She used to spent £30 a month on international dialling cards to ring Italy. Then Skype and WhatsApp came along and she no longer bothers. When data works in all the EU, you don't even need one of those card for the duration of the holiday. 3G/4G is the "over IP" of the telecoms world. Why bother faffing with GSM etc. services to make a call when you have a direct Internet connection on each device and can use any service at all?
It might also mean that my satnav picks up some decent traffic information when abroad too, rather than just assume everything is okay from Calais onwards because it can't get a data signal.