I bought the ratio pump from Velocity and have been very happy with it. It makes mixing odd ratio epoxy very easy. The EZpoxy that comes with the Velocity kit gets mixed in a 100:44 ratio. It would be a pain to use a scale to measure this since in the course of building I will go through many gallons of EZpoxy. The Velocity also comes with another epoxy that is mixed in a 2:1 ratio. I use a scale for this and it is fairly easy to get that ratio right, but even that simple ratio takes longer using a scale.
The pump is expensive, and I understand people not wanting to lay out $250 to simplify a task that can be done with a $20 scale... but the ratio pump works great and saves a lot of time. Just as any other high priced tool, you can spend time working without the tool, but it costs you time...how much is your time worth?