How much better? Enough that you could spend your money literally anywhere else for a skimmer providing it's recent and come out in front - regardless of how little you were spending. Something like a Bubble Magus NAC-7 is an excellent example - they were retailing for about $115 and (on an appropriately sized system) maintaining water quality in some cases significantly better than skimmers from half a decade ago that were around ten times that figure.The main question is tang lover and nork, how much better and how do you define a skimmer that is really good? Whats so good about them?
As to how to define what is good? Quite simple, really. Their ability to maintain an adequate level of fixation of dissolved organics and remove such wastes from a system's watershed. Also, the ability to do so in an efficient manner is a major advance. Needlewheels are now at a point where even the least efficient of which can process much larger quantities of water and provide a much more stable and higher rate of air for shredding. For a needlewheel design which most skimmers we have access to as hobbyists tend to be, these are both major improvements from the gear of old.
Skimmers have hit a point now where even the worst will still do the job at a reasonable level. "Better" is relative, granted, but the improvements are able to be mapped quite well and quite clearly.