I have been using the SkiVisions base flattener from Tognar for 5 years or so. It works quite well, but it is pricey and the bar shaped replacement blades are expensive. You can sharpen the cutting edges on the bar with a stone which comes with the tool when it becomes dull, but you need to be careful to keep the blade straight. I can get equally good base flattening results using a good scraper and various files, although it takes a bit more time. Either way, if I work at it I can produce a flatter base than I've seen with many stone grinding jobs. Flattening the base by hand is time consuming, but because it is a slow process you're not likely to ruin your skis. I tend not to trust the stone grinding process at most shops. An inexperienced operator can do a lot of damage in very little time.
My favorite way to use the Tognar tool is to make a few passes with the structure stone every few ski days to keep the base flat (assuming that it tends to become convex - ie: base high).