I recently switched from O2 to Three (whilst picking up a new iPhone 4S) and frankly it’s great. It must be great if I'm blogging about it! Key wins are as follows.

• All you can eat data with Three. O2 have recently abandoned unlimited data and charge based on fixed price data packages for paltry amounts – e.g. 500MB for £6 per month. To be fair I should point out that their systems told me I was only using about 200MB per month previously. But read on…
• Tethering! I can turn on Personal Hotspot on my iPhone and connect to it via Bluetooth, USB or WiFi from another computer and get all the wonders of the internet. This has revolutionised my train ride to work with my laptop. Phone stays in pocket and via Bluetooth I’m fully connected to the world. This is technically possible on some other networks but often only at a price, either to turn it on at all, or because you’ll be charged oodles for the data you use up.
• Raw speed. Over 3G I now regularly get 8MBit downstream, 1.5MBit upstream. That’s better than many people’s home broadband and a lot better than I ever got with O2.

It's not all fun and sunshine though. I found a couple of major downsides, which I will just have to live with.

• Visual voicemail doesn’t exist on Three. Frankly I was dumb-founded when I noticed this. I literally couldn't believe it, but apparently it's true. Instead you get a text message telling you that you have a voicemail and then have to phone a voicemail number and painfully navigate through things with the numeric keypad. It’s positively medieval and if I wasn't so smitten with their data service it would definitely be a reason to avoid Three entirely.

  Of course you only find this out after you've signed up because it's not something they point out to you. I have had conflicting messages from different Three employees in shops and on the customer service line about whether they ever intend to get with the program here. Alas I fear they won't.

• Customer service can be dodgy. It was a struggle to get my phone up and running in the first place due to some miscommunication and dodgy information from Three. Their customer service line is like a choose your own adventure book – consisting of endless numeric options designed to avoid having to actually talk to you. Once I actually got to speak to somebody it was alright, though I had to fight my way through the number-maze each time – they wouldn't give me a direct number.

  Before my phone was activated (which is ironically what I was trying to phone up to arrange) it was literally impossible to talk to someone because the first auto-question asked for your mobile number and then complained that it wasn't activated! Apparently you have to type hash to defeat this particular monster but they didn't tell me that and the prompt doesn't mention it. Apparently it's a secret.