IMO MathUPro is recommendable for straight calculations and programs. I used it on a Palm m515 and use it on a Pocket PC now. Fontwise, readability is very good for almost all labels and the display. You have 4 stack levels, but may also switch to 16 levels. With 16 you digest every formula in arbitrary order. MathU features by far enough registers and memory for your programming. The makers of MathUPro implemented improved program control commands, so you have better stuff than ISG and DSE. And you may fully customize your keyboard, i.e. set all 3 functions for any key (direct, blue & gold). Since your programs have alphanumerical labels, you may create your own commands and assign them to the (virtual) keyboard and they will show up there immediately. Last not least, since you have a backlight LCD, you may use it in the dark, too :)
What it has too less: keys.
What it does not have: e.g. complex numbers, matrices, regression, solve, integrate, named variables, menus and soft keys.
Since "it's only software" ;) , these points may may be corrected in the future, but I've no idea when.
What it cannot have: real keys with the nonpareil feeling of a true HP. And it will just work as good as your PDA in direct sunlight.
Summary: you get quite a bit for less than 20 Euros.
Edited: 26 Aug 2006, 6:23 p.m.