Hey Malteser
I think the EFIS add-on for FS98 is brilliant. It certainly adds a whole new dimension to the program, and it has lots of features too - cat1 autolands, APWS, and many different CRT displays. It also has extensive flight planning software and a well written and informative manual comes with it.
I cannot compare it to the real thing (as I have never used real EFIS!) but it certainly seems very in-depth, and I have learnt an awful lot about VORs, DMEs, fixes and other nav aids from it.
Airport 2000 is also another good add-on with lots of new aircraft types and detailed airfields, but as the saying goes, nothing compares to the real thing!