The Hechinger Report has a roundup here of the research on the effectiveness of Pre-K. It's a good refresher as more cities and states carve out funding for early education -- most notably here in New York City, where Mayor Bill de Blasio and Governor Andrew Cuomo have pushed to make Pre-K available to all of the city's public school students.
The big caveat here, as always, is this: It's all about the quality of implementation. Making Pre-K available is step 1. Step 2 is making it work.