The Xperia X1 is a strange fish and no mistake. Sony Ericsson has dipped a toe in the smartphone arena with this phone, and has come up with a wide screen, sliding keyboard handset with a superb physical design. All good.

There is a very original user interface too, which is pretty much a requirement for any high end smartphone these days. But underneath that interface Sony Ericsson has chosen to use Windows Mobile, an operating system with its roots in the business community rather than with consumers.

Build quality is excellent with a good deal of metal in the casing to help with robustness as well as high-class looks. The sliding keyboard has a lovely gliding action as it slides out of a long edge, and when it is in use the screen sits at a slight angle making it easy to view when the Xperia X1 is on your desk.

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Importantly, the keys themselves are well spaced and comfortable too; well, as comfortable as you are likely to find on a keyboard this small, anyway. Space for the QWERTY keys has been maximised which means there is no separate number row.

With the keyboard tucked away the Xperia X1 is all screen. With 800 x 480 pixels crammed in it is very sharp and bright. Beneath the screen the shortcut buttons include a D-pad with optical scroller (think iPod scrolling). You can turn this off if you'd rather not use it.

We mentioned an original user interface. This is based around what Sony Ericsson calls ‘panels'. The idea is that the main Windows Mobile look and feel is hidden away from view by a set of different main screens designed for different jobs. A range is pre-loaded and you can download more, though there aren't many available at the moment.

These panels show themselves in thumbnail view and you tap the one you want depending on the function you are after. One takes you to the FM radio, another to Google, another to the media files on the phone. While they look great thanks to the high resolution screen, in the end all these panels really do is act as an overlay to the features on the phone, and while some will see them as a nice way to personalise things, others will think they are a faff.

There are other goodies here that help justify the high price for this smartphone. Wi-Fi is built in as well as Bluetooth, there is an FM radio and the X1 supports HSDPA. There is a GPS built in too, and Google Maps is pre-installed.

The headset connector is 3.5mm and there is an SD card slot for boosting the generous 400MB of built-in memory rather than the more usual Memory Stick. These two features are probably more a function of the fact that the Xperia X1 has been made by Windows Mobile specialists HTC than it is an indication of a general change of direction by Sony Ericsson.

A 3.2-megapixel camera sits on the back of the device. It lacks some features we'd like to see in a smartphone at this price, such as Xenon flash and there is no front camera for two-way video calling. This isn't our main grumble though. That is how slow the Xperia X1 was at doing our bidding. Sometimes we found we had to wait for it to catch up, especially when choosing panels.

The Xperia X1 looks stunning and on paper it packs a lot of features. We like the hardware design and the high resolution screen. But the ‘panels' idea is a bit of a let-down and its slow running proved very frustrating at times.