Everyone probably already knows my oppinion of Proteus, but I'll say it again.
It's totally awesome. Best SIM I've ever seen. I'll never be without it again, unless I quit programming PIC's.

I just recently started using ARES (the PCB part) at work, and I've never made PCBoards so easily. It was actually fun for once. I always hated doing PCB's before.

It isn't exactly cheap, but if you can afford it, I would recommend it above anything else.

And NO, you don't need the "Advanced Simulation" stuff.
The real strength of Proteus is the simulation of your code in a microprocessor.
The analog parts work, if your design is good. Out of spec designs give results you wouldn't see in real life, so it's not a "Proof of concept" if you don't have a good design to start with.

It is invaluable for debugging code though.
Especially when you're debugging other people's code.