to refresh the post... I think the great thing is that Sensor has freeware version for educational purpose and up to 6000 cells (without visualization tools etc).
I used it on 2006, and it was very simple yet powerful simulator, and as long you don't care about importing from petrel or quick diagrams and 3D images, it's great.
I've been working in CMG, Eclipse, and Sensor so far and the Sensor is definitively the fastest.
It would be great to share some sample input files and to make short comparison of simulators.