I use Lee molds, they save a lot of money, especially if you are experimenting or are just getting started. I've noticed that when they get really hot, they tend to leak between the two mold halves. Usually I let them cool a bit and go right back to it. Never had any complaints about the overall quality of the bullets they drop.
However, I certainly feel that you get what you pay for with Lyman and RCBS molds (and other high quality iron block molds). Most of my "expensive" molds are slow to heat up, but once they get there, you can cast all day if your arm can stand to hold the heavy buggers up.
What I do sometimes is buy the Lee molds when I get some new caliber, then later order a Lyman or RCBS mold if I decide to keep the gun. It's nice to have extra molds for different bullet shapes, weights, etc to experiment with, so you cannot have too many different molds!