The calculator on the left is probably my preferred graphical calculators, it is a great balance of functionality and price. If however, you want the all-singing, all-dancing calculator then the one on the right is just a little bit better! (I have earlier versions of both and have used them regularly over the last five years or so. )
In all honesty, their usefulness increases if you are studying further maths, but they still have a role in the standard A-level maths course and with practice, they offer significant time savings in an exam setting. Details and example of their utility can be found in the various calculator sections of the website. In particular, have a look at the graphical calculator chapter for a guide to some of the common features of these calculators. A number of these features have now appeared on the class wiz calculator, which lacks the graphical functionality, but is still a pretty decent calculator.