A very good question. The answer depends on what all you use your digital diary for. The common features you have on a digital diary are 1)phone book 2)Planner/organiser 3)World time display 3)calculator 4)Memopad etc.
Lets get to the phone book part.
Are you the kind who carries a mobile phone? or does your watch have memory for phone numbers? If you are a digital geek, you are sure to be having either of these above. In which case you dont really need this option in your digital diary. My observation tells me that most people use the Digi-diary for the phone book and having these alternatives removes the need for a digital diary.
Second problem is the size of the digital diary. If all you need are phone numbers/email addresses etc, its much easier to carry a pocket phone book than to carry this bulky piece of equipment in your pocket. And provided you use a ball point pen, its much safer when you get it wet. dropping it too is without risk(im the kind who frequently spills the contents of my pocket on to the floor...)
The organiser:
A very handy tool. but how many of us use it? If ure the kind who uses it, then I would say, go for the digi-diary. But the ease of use depends on the make and the menu structure. the tiny pocket variety that is as large as a credit card is the toughest to program. but in general I found the Casio models and sharp models easier to use than other brands I tried (like royal etc..).
Memopad: Useful for writing short notes for recall later. Very handy but a useful replacement is a small pocket notebook(the paper variety) and a pencil.
Calculator: Most electronic devices including watches, cellphones now come with a built in calculator. Card calculators(yeah, those which look like visiting cards) are cheap too, and you just might find one for under 50Rs.
In fact in most of the higher end cellphones, you have all the features of your humble digital diary.
So the question...:::Should you go in for a digital diary?
The plus points: Cheap(you can get one for anything between 200 to 1000 Rs)
If its a good make, its bound to be easy to use too. You have all your stuff in one place, and you dont need to carry loads of notebooks and then transfer your data into other media (your comp/or regular diary etc)
Some makes (Sharp/casio and others) also come with software and hardware accessories to interface and link with your comp to synchronise data between your diary and your comp.
Points against: you can get yourself a higher end cellphone which would do all this plus give you the added luxury of instant communication.
You also have lower-end PDAs which are just slightly costlier but give you much more computing power.
The advice?:If you are on a limited budget and you have needs that are not yet satisfied by your current gadgets and gizmos(Phones/watches etc) then YES go for it.....
Else? you are better off with a pen/pencil and a pocket notepad.