Just thought I'd write up my experiences from a couple of months with a new Smartphone.
Why care about data usage ?
If you filled your car and due to a heavy right foot emptied your tank quickly would you go back to the garage and complain ? Or what if while paying for petrol another customer came in and wanted their money back because of the way they drove ?
That is the way some people think about data and there are loads of posts in the forums from people who have had their credit disappear. It is your responsibility to manage your usage. You have to decide where you are on the scale between being willing to think about data (and potentially run a cheaper tariff) or not caring in which case you’ll want unlimited data but be careful of tethering restrictions.
In a nutshell giffgaff allow tethering as long as you’re NOT on an unlimited data bag. Or to put it another way you cannot have unlimited data accessible to a device which isn’t the one with the SIM in it.
Update: Unlimited data is on the way out because, to put it bluntly, giffgaff never had this to sell in the first place.
This is written from an Android point of view though other mobile OSs do have equivalent features.
Get to know the “Data Usage” bit of your device. It’s under Settings under Android. Also how to switch Mobile data on and off, the meaning of “Sync” and the “Background Data” setting. Android Data Usage also has a handy limits feature.
giffgaff show you remainng data on your Dashboard page. My experience so far is that the Dashboard does agree with the phone to within 10% (for my usage pattern and an S4 mini).
The first time you fire up a new device (or one that’s been in the draw for a while) chances are it’ll have loads and loads of updates to do. These consume data quite possibly hundreds of megabytes. Ideally configure in WiFi as soon as possible. You really want to make sure the device gets all these update out of its system before enabling mobile data or even putting a SIM in. Often updates won’t be allowed to work over mobile data but this varies. (Google Play -> Settings -> Auto-update apps covers apps but not system software which is somewhere else.)
Before you go away on holiday switch off sync and mobile data before enabling Flight Mode. When you come back from holiday do not switch on mobile data before getting working WiFi unless you have a sensible data tariff. You may be able to make use of BTOpenzone/FON and others at the airport. April 2014 I used BTOpenzone in Gatwick arrivals without (my BT) credentials.
Other things I've noticed
Having Skype running eats maybe 5MB a day. As of recently by by default it does not keep an icon in the notification area to remind you it’s running. I suggest you re-enable this notification icon (in its settings).
Certain popular social apps update very very regularly which alone could eat a few hundred megabytes a month. As I said updates may or may not be allowed to work over mobile data. (Google Play -> Settings -> Auto-update apps.)
I found both Facebook and Google+ would upload any pictures I took supposedly for backup purposes. Thankfully there were notifications otherwise I might never have realised. Consider disabling these or at least make sure they do it only over WiFi. As well as data this uploading is not going to help your battery life. Weigh up against any need for pictures to be backed up quickly.
Update: Microsoft OneDrive and Dropbox also want your photos.