It's a retention mechanism for high values who only get phones from the shop, it's basically cash in hand for telcos and makes retention lockin easy so it's likely all will do it in time (other countries have had this for a while for what it's worth).
If we look at this from a numbers perspective, a phone a year is now something you can really only afford if you make a fairly decent amount so by offering it on credit you widen this to include people who have less disposable income. It also drops the wait barrier - turns out people are turning over at 18 months just to get a new iPhone and ditching the last 6 as effective writeoff. This is MONUMENTALLY stupid consumer behaviour but it makes the telcos money - if they can retain that is...
I suspect O2 did this first because they tried it out in.... I want to say Spain first but I can't remember if that's right. It was a South Europe trouble zone country and it did fairly OK with the target market (Young Professional is the target for the product, 25-40 mostly males) so they just copied it here.
So I guess the obvious question is the worth of it and I guess if you desire the phone enough as an upgrade then yes but with the lockin etc... I'm pretty sure I could do the cost out of contract for 2 phones / 2 years better than a network could do and save about 25-35% with cash upfront. But obviously not many people do have the cash upfront (and they arn't the target really, you'd already do this if you had the cash) so to them this is likely the better option.
Voda red hot I refuse to comment on, let's leave it at that