I haven't had any tuning issues, but I have had durability issues. I switched to Bowjax. They say they add 1 to 3 fps. They are much more rigid than the string leeches and last a ton longer. They have two kinds. One fastens over your string by way of cut ties. The other (the one that I am using) slips over your bow string. I prefer the second type and have been very happy with them.

I was happy with the performance of the string leeches, but the durability just wasn't there for me. The ends would get torn off after under 100 shots. I you tie them in, they will last longer, but it's just easier and cheaper to get the Bowjax.

Hope this helps you out.

Jim