Prior to this calculator, I used a Casio FX 991 ES (an old model). After losing it, I decided to upgrade to the ES-Plus. I was pleased to have upgraded -- the ES-Plus is a fully featured calculator yet the features are not too intrusive to hinder every day, 'casual' operations (such as simple addition).
When wanting to do something a little more complex (e.g. complex numbers, vector products, matrices etc.), a simple shift into a different mode and the calculator becomes even more powerful. A particular feature which is useful for the GCSE mathematician is the equation solving function (which can be used to check an answer after working it out by hand) -- this can be used to solve linear, quadratic and cubic equations. For AS and A2 mathematicians, the statistic functions (such as regression) are useful -- regression calculations can take a lot of working, so it's satisfying to be able to check that you're right using a calculator function! Complex/Imaginary modes and vector modes are particularly helpful in Further Pure modules. Another feature which I found useful is the 'table' function -- this allows you to calculate a series of values for a certain function in a certain range. I have used this many times in Further Pure 2 when having to draw polar graphs!
Arguably, the build quality isn't superb, but you have to remember that this is one of the cheapest calculators you can buy: and for how powerful it is, it is certainly worth it.