I have been brought up on just one, ECLIPSE so my opinion is suspect to start.

Schlumberger has bought up the best peripheral software to build the simulation model run by ECLIPSE for the widest set of problems commonly run in the industry. PETREL being one of the the "recent" BIG purchases.

On the ease of use, well here I heard that SHELL supposedly had something good in this department. However this is hear say. ECLIPSE is a sophisticated tool and requires a high knowledge from its user. The peripheral programs also have fairly steep learning curves. Hence it takes at least a year to close the build cycle regarding using the software (I saw this with pure novices). To get to expert level will take many years and will depend on the type of problems you work on.

Development of ECLIPSE is done by top people that SLB can hire. I suspect they do not cut costs here. This is bread and butter for GEOQUEST.

ECLIPSE is frequently a requirement for Development Projects farmed out by oil companies to consulting firms. So it is a Industry Standard Simulator.