Microsoft flight sim X is actually pretty good. There's load of add ons, so you're bound to find whatever aircaft you want.
I'm yet to find a sim where the visuals are so good you can seriously think about practicing VFR nav, even with the VFR scenery pack it's only a bit of fun, not for serious practice.
It is very good for practicing instrument work, particularly using the radio navigation aids (VOR, DME, ADF, ILS, GPS). These are usually skimmed over in an hour on the PPL course, but flight sim would enable a much better understanding.
The bad points - landing is quite unrealistic as you don't have the peripheral vision and It tends to lead to you ignoring the rudder, unless you invest in some pedals.