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Orange cranks up the data charges the minute you set foot on foreign soil. Data is charged at £5.50 per MB in non-EU countries, which means an email with a reasonably modest 2MB attachment will cost £11 to pick up in New York. Inside the EU, the Orange fee drops to an only slightly less punative £3.50 per MB.

O2 offers international data bundles that clock in at £20 for 10MB or £50 for 50MB, with excess charges in any country outside of western Europe costing a staggering £6 per MB.

Our advice? Leave the dongles at home unless you've checked the T&Cs very carefully, or if you have recently robbed Barclays.

Hidden charges

It's not only on foreign soil where extra charges come into play: stray over the tightly policed download caps and you're in for a rude awakening. "Some of them are very, very aggressive if you go over the limit," said Ablewhite. "3 charges 10p per MB, others are more aggressive than that."

He's talking about you, Vodafone. Stray a single MB over Vodafone's cap and you'll be charged an additional £15 per extra GB. That's double the monthly fee for its cheapest package.

In their defence, the software provided by most of the networks keeps a track of your monthly downloads and provides an onscreen warning when you approach your preset limit, which is all well and good if you're only using the broadband stick on one PC. If you're switching between different computers, however, keeping close tabs on your monthly consumption becomes much more of a headache.

The problem is exacerbated by software such as the BBC iPlayer, antivirus suites and Windows updates, which can quietly

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seep away hundreds of megabytes of data in the background over the course of a month. Turning non-essential software off while you're running mobile broadband is eminently sensible, although the peer-to-peer software used by the BBC, Sky, Channel 4 and others is notorious for continuing to bleed bandwidth, even when the client software is halted.

We suggest you pay close attention to your data usage for the first couple of months, until you can judge roughly how much you're using. The network's customer-service desk should be able to tell you over the phone how much data you've chomped through each month.

Image compression

One of the nastier surprises immediately apparent when you first start surfing with mobile broadband is image compression. They fail to provide any warning of it in their advertising, but the networks compress website images on their servers before sending them to your PC. They claim they're doing you a favour: compressing images artificially accelerates download speeds, and reduces the amount of data downloaded to help you stay within your monthly data cap. The end result, however, is web pages that look like they did in 1995: Facebook profile pictures are near indistinguishable, Flickr looks like a horrible experiment with an underwater Box Brownie and even the smartly designed banner on the BBC News page is mangled into a pixellated monstrosity.

You might argue that the networks are taking a diabolical liberty by compressing images on your behalf - especially Vodafone, which isn't shy about comparing its network speeds to home ADSL connections. Can you imagine the fuss it would cause if an ADSL provider decided to apply image compression? Unsurprisingly, it's a constant source of complaints on the networks' user forums. "I've just started using my USB modem on my iBook and iMac and the photo resolution is terrible on both machines," writes one unhappy customer on the Vodafone forum. "All photos on every webpage I visit and graphics seem blurred and very low res." And while some customers are happy to accept the trade-off, many claim compression makes the service unusable - especially if they're using it to update their business website or are professional designers.