Install once, update if you care to, but it's not essential. Requires no configuration after installation, works for ALL browsers on your system with no setup, does not require the browser to "support" it in any way (i.e., extensions), never ever gets broken by browser updates, works on ancient computers with grossly out-of-date browsers. Works with ANY tcp/ip-based app on your system, really, so it lowers vectors for IM apps, Acrobat, etc.
The first computer I used it on was an 800 MHz G3 iBook with 640 MB RAM. Some people may say a large hosts file will slow down your computer, but I've never seen that happen myself in over a decade of using it on literally every computer I have.
It may not block EVERY ad like a dedicated extension does, but it comes really really close, and I like the fact that it works with all browsers and never requires updating. When I get a new computer, I put the hosts file on and pretty much never touch it again. A handful of sites (like hulu) will not work with an adblocker and it's a manual process to edit the file, but for unix types, that's not a problem. It blocks google's sponsored links so you may need to take that out too, for people who google "sears" and click the first (sponsored) link instead of the first actual link.
No reason not to do security in layers and use it WITH adblocking extensions, I suppose, but I've never felt the need to.