Following news earlier in the month about mobile network infrastructure sharing, Vodafone and Telefonica (the company who own O2) have signed a deal to allow this to occur in four European countries which will see each company save hundreds of millions of pounds.
The countries included are Germany, Spain, Ireland and the UK with a possible extension to the Czech Republic and they plan to share sites, masts, and the onsite power supply. Each network will still maintain its own radio equipment and network for backhauling the traffic. The companies will continue to compete on retail business so there shouldn't be a huge effect to customers although coverage may vary slightly and customers may see some benefits from the cost savings in the current economic times.
I suspect initial reactions will be from people thinking this means their mobile phone signals will become worse, in theory some could experience better reception.
The other possible upside is a potential to reduce costs to us consumers and our phone calls, though that will probably be a while off yet.