We were lucky enough to acquire a Frasca 142 simulator earlier in the year. We use this for practicing the applied stages of instrument flight, for which it is superb. It is pretty useless for post PPL students who haven't yet started their IMC/IR(R) courses or indeed for those in the early stages of such a course. I'd strongly advise against any students who have yet to gain their PPLs going anywhere near such a device, even when the weather clamps in for seemingly weeks on end, as recently.
In order to become competent at instrument flight, I feel that you need to be able to fly in VMC and compare what the picture outside is like and what the instruments are showing you. Then develop basic selective scan skills, before moving on to the applied stuff, where a simulator might be beneficial.
As advised above, spend time reading and practicing test papers etc, ready for the exams.