Depends on the quality of the site and what their newsletter offers. As a general rule, I don't like pop-ups, but I also understand as a small business owner, why they sometimes can be useful if they are done tastefully and are not completely intrusive.
I know I hate it when pop-ups jump up at me before I have had time to read what I clicked on and then I will absolutely not sign up. If they are in my face as soon as I click on the site - nope, not going to bother. Other sites, you can read for quite awhile before a pop-up comes up and asks you to subscribe - then I will consider it if the content is truly helpful. I know that in doing so, I am getting good information and that site is getting support.