The most popular trend with zips in garments is the hidden or invisible type. In essence they are like most things brilliant when they are working but when they stick in seams or no longer work they need to be replaced and its a very common clothing alteration.
An elegant dress looks fantastic with a well sewn invisible zip but when there are a few layers of fabric or seams join they do have a tendency to stick and then break. Invisible zips are sewn very close to the zip teeth and can very easily catch on fabric causing the break or stuck situation.
The best advice is to buy quality garments and check how the zip runs before you purchase the garment (this might be why its on the bargain rack) but if its going to cost approx $25 to replace its a very good investment.
The other zip to avoid or be very wary of is the one that just wont stay up. Its lost its grip so to speak. Again just replace it and you might find a new wardrobe favorite or bargain from the end of season sales.