First of all, the locks meet their fundamental purpose. They hold my guitar in place and I personally did not have any accidents during my one year of using them.
Two minor issues that I found disturbing:
The screws wide enough (at least for my epi les paul custom) and the locks tend to get loose every once in a while (two/ three weeks). I try to make sure they are tight every now and then but this could possibly lead to the locks sliding out of the guitars if left unnoticed. An easy workaround to this would be to replace the screws with wider ones or to add wood glue for a stronger grip with the guitar's body.
Also, the locks sometimes make a squeaky sound. The lock itself is quite bulky and this can be quite noticeable when playing, especially when you move around. Sometimes this noise is heard through the amp as well. Other strap locks I have used on different guitars do not have this issue.
Overall, if you are not willing to spend more on strap locks, this product will work for you, but if the above issues seem important to you, you can probably get something that works flawlessly for a few more $$