i did a psychology experiment on the mozart effect at school, and had to research all the studies.
the jury is still out on the effects.
certain follow-up studies to the original mozart effect experiment claimed to disprove the original findings, showing that they couldnt be replicated.
however, there were inherent methodological flaws in these follow-up studies and also werent precise replication of the original experiment; they missed out several of the most important factors that supported the original work.
there have been some studies which do support the mozart effect and some which dont, about 50 in total now.
most studies have shown that the main area of performance enhancement has been that of spatial-reasoning abilities, which actually covers a lot of mental functions.
i wouldnt expect to find conclusive evidence one way or another quite yet, but do expect to find businesses jumping on the bandwagon for a quick buck, selling CDs like "Mozart for Maths" etc.
classical music is an incredible thing. it is some of the most beautiful music i have ever found. intelligence improvement aside, its worth immersing yourself in just for the pure aural luxuries it will bring you if you truly open yourself to it.