Size and shape. Is it small enough to fit into a pocket, or a bag? Is it ergonomically shaped, without sharp corners? If not, you may choose to leave it behind... which somewhat defeats the point of a personal digital assistant.

Weight. It may be small enough, but is it light enough (with batteries)?

Battery lifespan. Just hours, or long enough to keep going for days at a time? Notebooks are notorious for power problems. They have quoted battery lives of around five hours, typical real world life spans of three, and so they won't even last throughout (most people's) working day! Be wary of add-on modems which do not have their own batteries; they will dramatically reduce your handheld's batteries.

Reliability. Handheld software must be stable and seldom have faults.

Printing. Can you print directly to a printer? Or do you have to return to your desktop PC?

Speed. Is it fast enough? How long does it take to add a contact, or an appointment? If you end up watching an hourglass, the machine is too slow.

Keyboard. How large is it? How responsive? Does the handheld even have one, or is it a bulky extra? Handwriting recognition is only OK for quick notes.

Standard software. Is the built-in software sophisticated enough?

Is it aimed at 'personal information management' (a PIM / organiser)? Or does it have the breadth of a true palmtop computer? Which set of features do you need?

Is there a real word processor? With a spell checker, style sheets, fonts, etc.? Not just a version of Windows Notepad?

Is the diary software flexible? Can it repeat appointments automatically? Can you view your appointments the way you want to, not the way the machine wants to display them?

Is there a built-in spreadsheet? For graphing, and calculations they're invaluable.

Add-on software. How many titles are available? Do they cover your areas of interest?

Are they essential purchases (because the built-in software is limited)? Or are they genuine optional applications?

Not everyone wants to have to buy two computers when all they want to do is create and print out documents wherever they happen to be...

Can you make a safe copy of your information on disk, or on paper, without a desktop?

Does the software supplied talk to your application software? Can you exchange data with more than one manufacturer's products, or is it limited to just one company's software?

When you transfer documents, are all of the formatting options retained? Things like paragraph styles, fonts, formulae etc.

Does it link with the Macintosh?

Can you zoom the size of the text to make it larger for poor eyesight, or smaller to give you a good overview?

Is there efficient use of screen space? Or, is there useless 'screen furniture' which looks nice, but in practice wastes space and isn't useful?

Are there off-screen quick application access buttons? How large and wide is the screen?

Is the contrast good? Touch sensitive screens tend to have poorer contrast, and so backlighting is more important.