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When I put in my £15/mo 30 day sim and synced my phone, iTunes offered me a carrier update which enabled tethering.

Also, for those with a 3G iPad, Three are looking into possibilities of a shared iPhone/iPad data allowance.

No complaints with Three here in Belfast other than it can be a bit slow to switch over to 3G from 2G and no visual voicemail.
 
Yeah, I found it after a carrier update. I'm very pleased so far. The network speed seems substantially better than O2, and the new £10 tariff (100 minutes/1GB of data) suits me well.
 
AVOID

I signed up to 3 on iPhone 4 launch day and I really regret the decision. My signal is poor, I'm based in Uxbridge (West Greater London) I'm forever getting the "network lost" notification and the delay between switching from 3G and 2G is appalling. I'll also loose my 3G signal randomly despite the fact I can reactivate it by searching for cariers and selecting "3 (3G)" instead of just "3".

3 also filter a lot of internet websites which they class as inappropriate (adult themed, feature violence etc.) You have to pay £5 a month to turn off the filtering. MADNESS.

Customer service is VERY poor, I've given up already.

AVOID

Wow!. My experience is very different.

I moved from O2. I was unhappy that O2 reduced the data allowance without changing prices.

I found O2 3G reception to be patchy. With O2, I had no 3G at my house in Sheffield. I often got "phantom 3G". I was seeing two bars of 3G - but when I try to use it, the line degrades to GPRS. So I had had enough of O2.

As I understand it, in terms of network coverage, Three has the largest 3G network. I can now get 3G everywhere, including my house. I can get streaming audio in moving vehicles, on the motorway. I even was able to listen to streamed audio in a department store elevator. Something I could never do with O2.

Three has twice the data allowance - for £5 a month less. (6x if you pay the same!)
And it comes with with free tethering. Which O2 want how much for?
I just got tethering working with my iPad, thanks to iTether.

The content filter is (I agree) completely ridiculous. So I got it removed. After a lot of bitching, they removed it. I suggest going to their blog site to complain, and not to the stores or to customer services.

C.
 
I would recommend "3" as there 3G network is massive compared to "o2" (results posted of the ofcom website), I have an iPad on "3" and my iPhone on o2 and where ever I go with "3" constantly has full bars of 3G and it's always super fast around 2-3mb, but with o2 I would have to go into a town or large populated area to get 3G and it's never been faster then 1mb.
 
The only way to choose a network is to get a PAYG SIM and try it for yourself before committing to any contracts.

My own personal opinion, however, is that o2 and related "networks" are to be avoided. O2 has the worst 3G coverage (in terms of population covered - and was once told off by OFCOM for not meeting coverage requirements), and once you actually get a 3G signal, the speeds are utter crap (provided the network hasn't collapsed). They may have had 3 years to deal with iPhone traffic, but they still manage to suck.

I live in a rural area, and am with 3 (though not with an iPhone). I don't get very good 3G coverage where I am, but when I am on 3G the speeds are excellent. A personal best is 4.65Mbit down / 1.8Mbit up. They also have the advantage of having the best 3G coverage (along with T-Mobile, due to network sharing).
 
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