If you're using them properly with headphones on and eyes closed. Tiredness and headache are the side effects I've witness. I cannot say if any postive benefit ever resulted from their use in this way.
No recording that requires you to alter your physical and mental state (get physically and mentally relaxed) recommends more than 2 (broken) hours listening per day.
If you use them in the background not really going with them (repeated cycles in your sleep or running in the background while you are doing other stuff). The only side effect I consider likely is that they are less effective when you do want to sit down and get physically and mentally relaxed with them (you've managed to train your mind to largely ignore them).
When it comes to paraliminals I find them most effective using one only once when I have a specific goal or purpose... leave it for a couple of days to notice my results. I've notice that results don't come with a firework parade and are more often only to a level that I can handle it without 'seeing' it at the time. Usually I notice it later when I review a situation or day.
The harder one pushes to get something to work for them the further they push the desired results away.
Focusing and working on one problem at a time often starts resolving those other problems that bother us too. When we try to work on all of them at once we only wind up frustrating ourselves about what to do first.
Really for best results always leave 20 minutes between listening sessions. Use headphones and be with the tape physically. Also allow some time for your results to manifest. It's usually when you forget about it that it shows up 
Alex
