I bought the older version of this calculator 5 months before my GCSE maths exams. Even at this level I felt at a massive advantage: you can type in equations like 3y+1=5 and it will solve them. It also has a special mode which solves quadratic, cubic and simultaneous equations. Another time saver is when you want to repeat the same operation multiple times rather that scrolling along them replacing the number, for example if you wanted a list of what the value of a number cubed minus its value squared rather than entering 2^3 - 2^2 then 3^3 - 3^2, 4^3 - 4^2 ect. You could just:
1. Enter X^3 - X^2
2. Press CALC
3. Enter the value you want X to be and press =
4. Repeat steps 2 and 3 for whatever values you want
With a medium sized function which you need to input say ten values you could more than quarter the time.
I have just finished AS level maths and the new functions that are useful are non-symbolic integration and differentiation this means you cannot derive X^2 to 2X (calculators which do this are banned in UK exams) but you derive the function X^2 when X is 3, to equal 6. It also solves logs to the base of any real number as well as calculating the sums of sequences using the sigma notation.
The applied module it took was statistics and for this you can just enter the data you have been given and it is able to calculate the values for linear regression lines the PMCC, Sxx Syy Sxy, min max, mean, range etc. It can do other types of regression but I have no use for these yet, in fact I have not use for most of the features of this calculator and it's still incredibly useful, it can do so much means it's probably the calculator of choice from GCSE to PhD. Even though I have a graphic calculator I use this 99% of the time for shear ease of use.
Even when preparing for the non-calculator exams like some GCSE papers and core 1 (C1) at A-level this is useful as you can check your answers quickly or if needed do your homework very quickly on the bus to school.
I would like to say this calculator is no substitute for hard work and proper understanding of the concepts. While it is true you only will really be able to use this calculator with ease and unlock its potential if you could do a question without the calculator if required and I would only recommend using it as a time saver, especially if you want to do a maths related degree.
That said it can still get you marks where you shouldn't deserve them. At GCSE level they gave marks if the correct answer was seen even if no working was shown. So for one 5 mark question I did just that and I think got it right. Then at A-level I was stuck on one part which upon using the cubic equation solver showed me the correct method. These small marks could be the difference between whether a university accepts or rejects you, which for only [...] is a bargain.
Just to finish a note of the differences between normal and the PLUS version. I terms of build quality there is very little difference. The new one is slightly more stylish. Functionally the PLUS version will show the answer to quadratic, cubic and simultaneous equations in surd form which the older model doesn't, this can earn you marks on some exams that require a surd for answer for the next question.
This calculator is top of its range but lower models have features which this one doesn't: The two main cheaper models amazon currently sell (as of 08/2011) are the Casio FX-83GTPLUS Scientific Calculator
and the solar powered Casio FX-85GTPLUS Scientific Calculator
(also in pink) they are able to do prime factorisation, recurring decimal and verify inequalities. I am not sure why the 991 does not have these functions but perhaps it is so more useful features aren't compromised for calculations which can be done easily enough already.
If you are buying a calculator for secondary school for the first time buy this, but don't exploit it. If you already have a basic scientific calculator already then keep it but buy this one or a newer version before your first important exams.
2011 UPDATE: Having completed my A-levels I found that for Core 4 the vector mode was useful. If you do further maths then this calculator is extremely useful as it can do complex number calculations including squaring and cubing in a way that I find much faster to enter than on a graphical calculator (but it cannot do square or cube rooting). It is also able to convert between algebraic and polar form, in other words find the modulus and argument of a complex number. Finally it has most (A-level) Matrix functions such as inverting which are useful to check your answers with.