Depends on what you mean by effective. This article summarizes and links a few quality studies.

For symptomatic Omicron infections: 2 dose ~50-60% effective, 3-dose ~70-80% effective, 4-dose ~90% effective. The death rate was not reliably calculable in these studies because it was so low for all groups. So, is 50% efficacy against symptomatic infection, [some very high efficacy]% against death/severe illness "effective"? It certainly slows the spread and keeps a lot of people alive. Plus there's strong evidence you could "choose your own efficacy", if you were higher risk or just didn't want the hassle of symptomatic Covid, you could choose first and second boosters.

For reference, the flu vaccine, which is becoming a better comparison as we have vaccines against this family of coronavirii and they have become endemic (there's a smaller and smaller population with no prior immunity) is typically 40-60% effective against the most common strains of flu each year.