Yeah, phat summed it up pretty good. Cheap tents will leak.

To add on to what he said you also need to take each individual situation you are in into account. For example, back when I used to carry a lot of weight I was in my Sierra Designs Meteor (this is a decent quality "bomb proof" tent) in a monsoon one time and it "leaked". Well, actually it didn't leak. What happened was condensation built up on the underside of the fly, and started dripping down into the tent through the mesh parts of the tent. Unfortunately on this coast you can't get away from this (it's just humid over here).

So site selection and condensation control (like wiping the fly down with a pack towel occasionally) are the best ways to deal with water when it is humid and/or raining.