For practice i.e. you already hold an IR or IMC then they are great.
Even FS 2000 can be used to keep oneself up to speed on the various procedures...sids, stars, holds, approaches and more importantly, missed approaches.
However, no matter how curent you are on the clomputer, this does not have any gain over the minimum aircraft time required to be "current" as per the AIP.
The best use for FS2000 is to fly the route and procedures in advance. Thus if planning a flight IFR from say Bristol to Liverpool, in the day/ days before, one can practice the separture procedures, route and approach procedures including missed approach. This gets some of the important figures into the brain (just like doing the test routes on the IR course) and vastly increases situational awareness.
DFC