"So tell me!", I hear you ask, "What are the advantages of using a slide rule?"

Personally, I do not own a calculator as in the ones where you press all the buttons and something cute appears on the display, although I have used them due to their compulsory requirement when I was teaching.  It seems that in the educational circles of mathematics, the calculator is one of and probably the only form of technology that is used in many schools.  Computers take pride of place, but even they are often used as glorified calculators.

With the apparent never-ending development of computers and calculators, it seems that our students need to learn or be capable of doing less and less.  Only one problem ...  What happens when this type of device is taken away from them?  Will they resort to paper and pencil techniques? ... Doubtful because few of the younger generation know the techniques well even to survive.

Okay, I know that even the slide rule could be classified as a calculator of sorts, but there is a real and distinct different between a slide rule and a calculator apart from the obvious.  Now some people think I'm a bit strange for using a slide rule and now that I've acquired my new pocket rule (a spankin' new Pickett N600-ES and Sun Hemmi 260 thanks to Dick Rose of Vintage Instruments) to replace my older and not so powerful Aristo pocket rule, I can perform all manner of calculations at anytime without diving for my British Thornton AA010 (although it's still one of my favorites, but hey, the Faber-Castell 2/83N is winning me)!

So let's look at some disadvantages of the new breed of calculator ...

1. Probably the most obvious is that students simply press buttons which correspond to the problem they're doing at the time.  For younger students this does not encourage or teach any significance of size when inputting numbers.  In most situations, students have not been shown any real technique to check the validity of their answer (apart from re-inputting the information again) and so have no real idea whether the answer that has appeared is correct or not.  In most cases, students will assume the answer to be correct regardless.
2. These new computer-type calculators, like the new computers themselves, can basically teach the steps required to find a solution to a problem by a series of prompts.  So there is no need for students to remember any techniques.  Lack of memory power!  I often found that students were extremely reluctant to even try to remember methods when they knew that their calculator was capable of doing the problem.
3. Simple, basic mathematical facts that were once thought to be necessary like the multiplication tables, fractions, ratio and proportion are far from satisfactory!  Why?  Because the calculator does all this.  It is not uncommon to find senior students who are extremely slow at reciting the twelve-times table, if indeed they can get it right!
4. The calculator has become like a security blanket to many and in some cases, the pocket computer.  Look out!  Battery life is only for so many hours ...

Now let's look at these in comparison with the slide rule ... and what some may call disadvantages.

1. First and foremost, you cannot simply punch the information into (or onto) a slide rule.  You actually need to know how one operates.  To some this might be what you call a disadvantage of the slide rule.
2. You will need to know some rules of basic mathematics and some things such as scientific notation and laws of exponents.
3. Accuracy can only be gauged to around four significant figures.

Can't think of much more.

Okay, now the advantages and I think the advantages will far outweigh the disadvantages!

1. Well, slide rules are extremely portable.  They fit into a brief case no worries of clip into your pocket.
2. They do not require a source of power.
3. You don't have to worry about frequent updates to correct potential problems or security loopholes.
4. It is an educational instrument that will help reinforce other mathematical properties and laws by constant use.
5. It help keeps the brain in tune.
6. In a learning situation, it teaches a better idea of scale and fractions, something students are very lacking in.  How so?  Because you need to be able to read a scale and then make accurate judgments about setting the slide.
7. It can always be used where high precision is not required, but let's look carefully at this one!  Of what practical use is it to be able to calculate results to the nth decimal place or n significant figures?  In reality, not much at all, after all, we put man on the moon with the calculating power of slide rules!  Living in such a macroscopic world, the slide rule sort of has a built-in error for things to be constructed. 
8. Exactness is not always that good, required or desired.
9. Compared with a normal calculator, the slide rule in the hands of a experienced user, can calculate the result very quickly because the scales on a rule work in conjunction with each other.