Overshoe for Specialized Sport MTB Shoe
After many years of riding in foul weather, I rarely use overshoes for above freezing temperatures.  They are NEVER waterproof.  The opening for your cleat and heel allows water into the setup which inevitably makes it's way up to your ventilated upper.  For them to actually be waterproof, they would need to seal to the bottom sole of your shoe.  Additionally, the cleat attachment area of your shoe would need to be waterproof (many are not).  More than occasional walking tends to wear them prematurely.  I've owned several brands and never had a set make it through more than three seasons (fall, spring and perhaps part of another fall) before they are coming apart at the seams.  

Several companies make foul weather shoes.  They range from truly insulated winter boots to versions of the companies regular offerings with no ventilation holes and a sealed bottom around the cleat area.  I've had several pairs of these from various companies and all have lasted longer than 4 seasons (2 years).  

In my opinion overshoes are only appropriate as an insulation layer when it's too cold to be wet or when you are a pro doing a time trial and they extra 0.75 seconds they afford are worth it (they are free from your sponsor at that point anyway).