O2's Xda line-up of Windows Mobile Pocket PCs has been around for a few years now, and the operator regularly adds new models. The latest to hit our testing bench is the Xda Argon, an update for the popular Xda IIi.

The Xda Argon is unusually large and heavy (200g, 129.7mm tall, 69.8mm wide, 18.8mm thick). That will irritate those who like their gadgets to be tiny but will please anyone who finds reading the small screens of Windows Mobile devices a bit of a chore, because the Xda Argon has a screen that measures a massive (in Windows Mobile terms) 3.5 inches corner to corner.

Such people will also probably like the fact that O2 has opted for the relatively large SD card format over the tiny teeny miniSD and microSD choices that are popular for PDAs and phones these days. It may be a while before you need to resort to extra memory though, because the Xda Argon has 256MB of ROM and 128MB of RAM, which is generous in Pocket PC terms.

Bluetooth, infra-red and Wi-Fi are all here, there is a 2-megapixel camera, and a competent 400MHz processor manages to keep the device going effectively. Battery life, too, is good. We asked our Xda Argon to play music off a full battery charge and leave its screen on throughout, and it obliged with tunes for a few minutes over nine hours.

All this is good stuff, but in a couple of respects the Xda Argon might not impress quite so much. First off, consider the operating system. Windows Mobile 5.0 is fine as far as it goes, but Windows Mobile 6 goes further, and those who want to use their Xda within a corporate network driven by Microsoft Exchange Server may feel short-changed by the absence of Windows Mobile 6's up-to-the-minute extra features.

Then there is the fact that the Xda Argon is tri-band GSM where other Windows Mobile Pocket PCs manage quad-band and some throw 3G into the mix. Finally, bizarrely enough, there is the size to take into account. Yes, the overall large dimensions will suit some users, but others will hanker for one of the many smaller format devices, usually about a third smaller.

We can't help thinking O2 could have tried a little harder with the Xda Argon. We are sure some people will like it, and it is an obvious upgrade for anyone currently using the Xda IIi, but if we were looking for a new Windows Mobile device we'd be sorely tempted to go for something smaller and more feature-packed.

O2 has trodden a fine line with the Xda Argon. It is good enough to encourage those using the Xda IIi to upgrade, but may have trouble convincing anyone coming to Windows Mobile for the first time to choose it unless they have difficulty getting to grips with the many available, smaller-screened, more pocket-friendly alternatives.