All hail the rise of the smart phone. This new breed of mobile is, as the name suggests, rather more clever than your current mobile phone (and a little more bulky and expensive too). But, according to technology research company Canalys, smart phone sales will overtake handheld computer sales in Europe this year.

On the face of it, you would hardly find this surprising. Just think of their respective prices: the Orange SPV (a smart phone) sells for £180 - well below the £400+ price level many people assume a handheld personal digital assistant (PDA) would cost.

In fact, you do not need to spend that much to pick up a PDA any more. Recent falls in the prices of PDAs have been lost, it appears, on those with whom I have conducted a quick straw poll last week. The fact that you can pick up a Dell Axim PocketPC for £197 surprises many. Jack Schofield wrote a comprehensive roundup of new PDAs in last week's Online; his piece showed that the Dell machine is not an isolated case either, since Palm lowered the price of its Tungsten T machine to £300, and the best looking machine of the bunch, the HP iPaq, now costs £299.

These price cuts, coupled with reluctance from mobile phone operators to subsidise their smart phones, mean there is substantial crossover in the price bands of PDAs and smart phones. The Sony Ericsson P800, for instance, a new smart phone that has created quite a stir, is being sold by Orange for a princely £309, which takes it right into PDA country.

So why are we still opting for smart phones, instead of considering the bigger-screened, more powerful PDAs? Two fundamental issues - form and function - continue to create a headache for the PDA.

First, form. Unless you are specifically out shopping for a PDA, you are likely looking to replace your mobile phone.

And the truth is PDA's don't make great phones. Some don't have telephony built in. On those that do, making a call is a cumbersome experience. PDAs are more bulky than a "normal" mobile, too: while they might be OK when you are going to work, and so carrying a bag (or wearing a suit), you'd probably not take one down the pub or out clubbing.

Smart phones are also bulkier than standard phones (should we now call them dumb phones, following the pattern set by smart and dumb bombs?). But, because they sport smaller screens, they are still phone-shaped, and sized, and live comfortably in a pocket or small handbag.

Then there is the question of functionality. PDAs, especially with the arrival of Microsoft's PocketPC operating system, pack in a lot of features. You can edit Word files, open up Excel spreadsheets and view multimedia files, as well as indulge in the more mundane businesses of email and diary management.

But how often in our lives is the need to update a spreadsheet so urgent that we're willing to do so on a tiny screen, using nothing more than a stylus and the rather erratic handwriting recognition built in to these machines? If it were something that happened at all regularly, we would be happier with a laptop.

For most of us, there's no desire for any of this kind of functionality in the first place. Trading in our current mobiles, we might consider a camera phone with a nice colour screen and some polyphonic ring tones, and those information services that used to be called Wap.

But we don't want to manage our diary on a handheld, fiddle with Bluetooth connections or surf the web while we're on the move, and we'd really rather not spend more than £100, thanks.

So where does all this leave the PDAs? Threatened at the top-end by laptops more powerful than they will ever be, with smart phones homing in on the "road-warriors" that used to be the PDA's home ground, and with feature phones mopping up the mass market, it seems the handheld computer is striving - and failing - to be all things to all people. So the question still has to be asked: what, exactly, is the handheld computer (really) good for? Too many shoppers appear to be giving the obvious answer: absolutely nothing.