How many use them? We have lived without them for 20 years, but as we get older and less agile, perhaps we should fit them. The advantages seem obvious, but do they really save much effort? Are there any disadvantages, such as chafe, windage, etc?
Ok, a post a little late is better than never… I had lazy jacks on my last boat and found them to be a bit annoying… mostly for two reasons. The first is that I had a full battened main, and the battens would constantly get caught on the wrong side of the lazy jacks when raising the main… so it was raise, lower, raise, lower, wait for the right moment, then raise again! It was a pain. The second is that my sail cover was not cut to include the jacks, so they had to be stored up against the mast. Not a huge deal, but it was one more thing to deal with… puling them out and setting them when the mainsail was uncovered, putting them away before putting on the sail cover.
I ended up removing them and installed a different system, something I read about but don’t remember what it’s called. It involves attaching short lines between the reef points (I had three) and the backstay. When the sail is aloft, they don’t affect the shape of the sail. But when it’s dropped, they help hold up the leach and keep the sail from spilling over. Not as neat as lazy jacks, but good enough, and I thought the trade off was worth it.
I have lazy jacks on Lyra, though they only come down to two points, the after most a little past halfway down the boom. I’m thinking of adding a third. Mine stow forward to the mast when I put the cover on the sail, and then when I raise the main, I simply don’t deploy the lazy jacks until the main is up. That way I avoid the problem that Tim mentioned about the sail catching when you raise it. I’ve found that, as a single hander, they help keep things under control when dropping the main.
I like that process of deploying them after the main is raised and stowing them before putting on the cover. In retrospect I think mine were not well setup for quick and easy stowing, solving that problem might have been better then simply removing them 
Though it seems you could still run into batten issues when removing a reef.
I haven’t yet had reefing trouble. I think the first reef has the head of the sail well above the anchor point for the lazy jacks. That may also be the case for the second reef. I’ll keep an eye out for it, but in any case, it’s easy to pull the lazy jacks in before raising the sail further (and now that I’ve written that, I remember that I’ve done it). When I’m shaking out a reef under way, you only need to pull one side in (leeward).