ti-89

this is quite simply the most impressive calculator i have ever seen. for a machine of this complexity, the choice of keys is very good. the keycaps themselves appear to host strange and rare things, but in fact, these are the things you need when working with formulae. for example, x, y, z & t are on keys of their own, other letters are on shift keys. formulae mostly involve these variables so it make sense to put them up like this (unlike hp).

standard scientific functions are also on the keyboard, albeit on shifted keys, but at least they're not hidden away in menus (like hp). the problem here, is when a calculator acquires the ability to perform rare and wonderful functions, you must not lose track of the fact that the mostly needed functions are still the simple ones and they need to be accessible.

the drop down menus are easy to use with the f1-f5 buttons and the 4 direction keypad is just great to navigate the screen, scrolling and even cut and paste from previous expressions into the current one. which i like a lot. although the case is somewhat plasticy (hp wins here on build quality with the 48g), the rubberised buttons work well and feel positive. note here that its a different story with some cheap ti's like the ti-30xbii. 

calculation in general and symbolic operation rocks, humiliating the hp offerings. for example, general (numerical) calculations are first handed to the symbolic engine and then numerically evaluated. this makes for partial simplifications and greater accuracy. this works well for stuff like solving and definitive integration where the integrand sometimes gets modified to accelerate the numerical process or sometimes solved analytically for a complete symbolic answer. check some out in the torture test. even raw numerical accuracy is excellent. its also fast.