- Accuracy: How accurate it is ?
- Speed: How fast it run?
- Capacity : How complex board can be supported ?
- Compatibility : How many layout database supports?
- Integration : How well it get integrated with your design flow?
- Support: How well the support be?
- Value for the dollar: How much it costs?
The priority of these seven attributes really depends on what you are designing.
Above is what Daniel comments on evaluating Spice simulator on SEMIWIKI. I agree it and think it's true to evaluate a SI/PI tool(The underlines red was added by me). We don't see there is a holistic solution to cover above aspects well.