Its a development suit, not just a standalone package;
You get the most powerful version of basic designed and continually updated for both the intermediate and high-end users;
Free updates, and there are many 'addons' that allow simple interfacing with SD cards for example, I’m just touching on the basics here
A VSM (to simulate your circuits in near real time, although its only locked to certain configurations - but its damn handy anyway, and the full version is probably the most powerful VSM on the market.
In Proteus 6 Professional you can develop complete circuits with thousands of components, including digital devices running with I2C/SPI/DALLAS 1 WIRE/UART practically anything, and then throw in a few virtual terminals for real time debugging, dc/ac signals for ADC/DAC/anything, and as soon as you hit 'simulate' your circuit is as it would be in real life.