For two years I used T-Mobile, once that 2 year contract had completed I then tried Giff Gaff and the unlimited data plan.
I still like Giff Gaff and the way they operate, and would recommend them to most people, but there was a big problem for me.
Giff Gaff are an MVNO on the O2 network and when in town or most cities the data service was good and at a reasonable speed, but I do a lot of wlking and hiking out in the countryside and in my experience the data speed was unusable and could not be relied upon for anything more than low latency data.
I gave up and moved on when I discovered that Three offered, for £12.90/month offered Unlimited Data, 200 minutes and 5000 texts, and all on a contract that was just one month long, having just purchased a Nexus 4 that was compatible with the top speed on their ultrafast network it was a simple choice.
Out and about in the Southern English countryside I cannot recall that the data speed has ever let me down, the reception is a bit patchy in the house, but when its connected it beats the O2 signal hands down, and I regularly get download speeds of 20Mb. Even out in the countryside when there is a signal, and there usually is, its at 3G speeds.
After less than a month I contacted three and moved up to the £25/month one month contract which has oodles more minutes, but more importantly allows unlimited tethering for other devices, such as my Nexus 7 and my Chromebook. The service that I get from the network has made the Chromebook a useful device on the road, as quite often I give up on public WiFi as the connection has often been significantly better on mobile.
Two travels abroad, one to Italy and one to France and Germany, using three's Euro internet pass of £5/day unlimited data throughout Europe keeping me connected (even crossing borders) especially as there was no open WiFi available in the village we were staying in made life a lot easier planning day trips on public transport, and finding those hidden gems.
Annoyingly just after I returned from Italy Three announced their feel at home service that allows you to use your phone in the seven countries that Three have a presence, including Italy, as if you were at in your home country with no added costs.
Looking to the future Three UK have promised when they roll out their 4G coverage at the end of this year there will be no additional costs.
This is why I have decided that Three UK is the best fit for me.
p.s. in writing this post I note that three have lowered the price of the one month one plan back to £18, maybe time to ring customer services again.
Update 21/9/13: contacted three about the £18 price plan, which is exactly the same as the one that was costing me £25/month and changed, apparently this is a special offer at the moment, as well as the 12-month plan that is £15/month for the same.