i wouldn't bet the farm on a sticky board mite count, however....the number of mites on your bottom board does give you an indication of your mite load. even if those that drop are unhealthy, if you have a lot down there you can bet that you have a bunch in your hive. on the other hand, i have never found a huge mite load in a hive that didn't have a pretty good mite drop.
i like the slide in board in my screened bottoms for just that reason. they are easy to pull and take a quick look. what i find is one part of the evidence i use for decision making.