I have a Simplicity brand serger which is about 20 or so years old. It's been nothing but a headache nearly every time I try to use it. On the rare occasion I get it working well, it gets all messed up if the thread breaks or I have to change colors. Even if I tie on the new one to the old one, it still doesn't thread right. I have had it in the shop a few times. I think if there is any slack in the threads, they cross over wrong. I'm not spending any more on it. I wish I could get it working...it's nice for making curtains. If I ever got the desire for another, I'd buy the self threading one. But they are pricey, so that's likely not going to happen.
Be sure you try it out before you buy it. Try googling for sewing machine reviews also. It's always good to read about the particular brand/model to get feedback from owners.