BlackBerry-style mini-QWERTY keyboards are still common, but smartphones with slide-out keyboards are relatively rare these days. Examples we've reviewed this year include the Sony Ericsson Xperia X10 Mini Pro and the Motorola Milestone, and Nokia's forthcoming E7 will be a notable addition to the genre.

HTC clearly feels there's life in the slide-out keyboard, and its new Desire Z continues a tradition for the company stretching back to the TyTN in 2006. The Desire Z is HTC's first Android-based phone with this form factor.

Design
The most immediately noticeable thing about the HTC Desire Z is its weight — 180g, which is a lot for a smartphone measuring 60.4mm by 119mm with a 3.7in. screen. The sliding keyboard is the reason for the weight of course, and for the handset's chunky 14.16mm thickness.

The 3.7in. Desire Z is HTC's first Android phone with a slide-out QWERTY keyboard

The use of high-quality build materials also adds to the weight, although the metal screen frame and battery cover also add solidity. The three-way colour scheme of black, grey plastic and lighter grey metal makes a welcome change from the uniform black of many smartphones.

The most interesting aspect of the Desire Z's design is the keyboard mechanism, which raises the screen section as you slide the keyboard out. Only a slight push is needed to activate the mechanism, although this can be drawback at times. With the keyboard open and the Desire Z on a desk, it's tempting to move the device by grasping it by the long edges. However, doing so will often close the device, which isn't necessarily what you want.

The main benefit of HTC's hinge system is that by raising the upper section and then lowering it to meet the keyboard section, the keyboard can rest on a platform. This helps avoid the problem of the keys sitting in a hollow that can make the upper and lower rows awkward to access.

The keyboard has four rows, with the number keys embedded in the top row and accessed via two Fn keys — one at each end of the third row down. There is a Menu key, and two user-configurable keys for apps or shortcuts. The @ symbol has its own key, at the top left where you'd normally find the Esc key. Also, the €, $ and £ symbols all available via Fn key combinations — often at least one of these is hidden away in an on-screen keyboard.

Additional symbols are available if you give a key a long press. Often these replicate what you can get with Fn key combinations, which is a little confusing, but they also give access to a range of non-UK alpha characters. Hold down the 'e' key, for example, and you can choose from é, è, ê, ë and so on.

Despite these positive features, we found the Desire Z's keyboard less than satisfactory to use. The individual keys are well spaced, but they're not as tactile as we'd like. We managed about 70 percent of our maximum speed with a slide-out keyboard.

The capacitive touch-screen accommodates the usual on-screen keyboards in portrait and landscape formats, and we found the latter just as easy to use as the physical keyboard. The main benefit of the physical keyboard is that it frees up screen space: for example, you can see search results as you type into the Google search box, and more of the email software you're tapping into rather than just a simple text-entry box.

The 3.7in. screen has a resolution of 480 by 800 pixels, which is the same as the original HTC Desire, the new 4.3in. Desire HD, and a number of recent Windows Phone 7 devices, including the Samsung Omnia 7, HTC 7 Mozart and HTC HD7. The display is sharp and clear, and the screen responds well to the touch.

Beneath the screen are four touch-sensitive buttons with the usual Android functions: Home, Menu, Back and Search. There 's also an optical navigation pad with a press-to-select function.

Elsewhere, there's a 3.5mm headset jack and a power switch on the top, a volume rocker and microSD card slot on the left, a microphone on the bottom and a camera button on the right.

The HTC Desire Z ships with an AC adapter, a Micro-USB cable, a stereo headset with flat in-ear buds, a printed quick-start guide and an 8GB microSD card.

Features
The Desire Z's 800MHz processor might seem underpowered compared to cutting-edge rivals with 1GHz CPUs. However, in practice this Qualcomm Snapdragon 7230, backed up by 512B of RAM, delivers performance on a par with the best we've seen from today's top-end smartphones.

There's only 1.5GB of internal storage, but this is augmented by an 8GB microSD card. Unfortunately HTC has located the microSD card slot underneath the battery, so swapping cards in and out will require a reboot.

For wide-area connectivity, the Desire Z has HSPA supporting up to 14.4Mbps down and 5.76Mbps up — network operator permitting, of course. Wi-Fi (802.11b/g/n) and Bluetooth (2.1+EDR) are also present, along with GPS. Google Maps is preinstalled, photos can be geotagged and Latitude allows for location-based mapping services. As well as Google Maps there's a Car Panel that offers a similar look-and-feel to personal navigation devices; you can buy premium mapping services with local caching of maps.

The Desire Z runs the latest Android 2.2 operating system, which includes features like operating the handset as a mobile Wi-Fi hotspot. The HTC Sense 'user experience' is overlain, as usual, allowing you to populate the seven available home screens with an array of widgets and shortcuts.

For the first time, HTC Sense can run in wide-screen mode. You can also share videos, photos and music via DLNA with other compatible devices.

The Desire Z's 5-megpixel camera supports 720p HD video recording — not something we're accustomed to seeing in an Android handset. The camera has autofocus and flash, and a couple of effects we saw for the first time in the 8-megapixel Desire HD: distortion, vignette, vintage and depth-of-field.

The HTC Desire Z is compatible with HTC's new Sense.com service, a set of cloud-based tools. Among its features is remote access to your handset to make it ring (handy if you can't remember where you left it), to locate it on a map (handy if you think you've lost it) and even to lock it and post a reward message on the screen (handy if you have really lost it). As a last resort, you can remotely wipe the phone's contents from your HTC Sense.com web page (handy if it's lost and you're not going to get it back).

HTC Sense.com also offers a complete SMS archiving service, a Footprints archive, a contact manager with handset syncing and a subset of apps from the Android Market that HTC approves of.

Performance & battery life
The Desire Z's 800MHz processor handled all of the tasks we performed during testing without any problem. However, battery life — as ever — is a concern.

The Desire Z has a smaller screen than the Desire HD and a higher-capacity battery — 1,300mAh compared to 1,230mAh. Consequently the Desire Z lasted longer than the Desire HD, and we were able to get a day's life from it under a moderate usage regime. You'll still need to charge once a day though, and if you're a heavy user of battery-hogging features like GPS and DLNA you may need a power boost during the day.

Conclusion
The HTC Desire Z is a relatively bulky and weighty smartphone thanks to its slide-out keyboard. We found the on-screen landscape-mode keyboard just as easy to use as the physical keyboard. Not everyone will agree, though, and we suggest you try out the device before buying.

Keyboard aside, there's little here that we haven't seen from HTC before. However, the range and scope of the included apps, HTC Sense and the new cloud-based Sense.com service make this is a very capable handset right out of the box.