I was finally able to see this after reading about it some twenty-five years ago. I still haven't read George V. Higging's novel the film is based on, but I intend to - one of these days. I know some of you are gonna say "You're looking forward to a treat" or some such, and I'm sure you're right. No question about that.
As the film is pretty great. The laconic style is familiar to anyone who's seen Bullitt, made by the same director Peter Yates, but there's not much talk in Bullitt. The Friends of Eddie Coyle, however, is very dialogue-driven. The characters just talk, talk, talk - and it's all fascinating! The lines don't all make much sense and they don't forward the plot, but it's much greater for that.
The film also trusts that the viewer can detect important plot points for him/herself. There's no one telling what happened or what will happen. In this respect, the final scene may be unnecessary.
The minus point in the movie is the funky soundtrack that's ripped out from a blaxploitation flick. This could well do without a music at all. (Mind you, Dave Grusin's soundtrack is very good, but not in this particular film.)
Here's a New York Times run-down on Higgins.
More Overlooked Movies (later, hopefully) at Todd Mason's blog here.
Edit: seems like Todd hasn't made up his usual round.