Tools, Sharpening and Power Machinery - vacuum sandblaster - opinions

I have a chance to pick up an older model vacuum sandblaster, but I need opinions on this thing first. It is basically a regular siphon-feed blaster and vacuum cleaner in one unit. The gun is a combo assembly of a normal gun with a vac nozzle and hose attached in the front. I guess the idea is that once media bounces of the workpiece it gets sucked into the vac along with dirt and debris, thus leaving less dust flying around and allowing for an indoor operation. But I have some questions about the whole concept..
Like, doesn't the vac decrease the force of the air/media jet? Doesn't it vac up some of the media before it hits the workpiece? And what about visibility when you work, it seems the blasted area is covered up by the wide vac nozzle, how do you know when to stop?? If anyone has experience with one of these, I would appreciate you sharing it with me.