I've given them a try. Really can't say how well they work. I've gotten trail camera photos of deer smelling the dripper, and deer have made scrapes under them, but I've never seen a deer use one when I was around it. Guess what I'm trying to say is, I wouldn't buy one if your plan is to use it to attract deer like bait would, but if you just want to get deer to visit an area it will work just fine.
But then again so will a lot of other things that you can do for free. [:)]
As far as when to set it out, if you've got one I'd hang it now and run it through the season.
“There are two spiritual dangers in not owning a farm. One is the danger of supposing that breakfast comes from the grocery, and the other that heat comes from the furnace.”