Goodfellas was a good movie, but not a lot of interesting gunplay. It shows a true story, and shows the down/dark side of gangstering. I don't think it's worth buying (it's NOT if you'd be buying for gun action), but it's worth seeing for the Peschi character scenes, especially the "in what way am I funny?" scene with Peschi and Liotta (Liotta plays the main character, Henry Hill). Goodfellas is kinda like the scene from Pulp fiction, where they shoot the guy and get blood all over the place, and have the argument about cleaning it up - shows the ugly reality of that type of life. Both Goodfellas and Casino are extremely violent, to the point of grotesqueness, agreed, particularly Casino, but Casino is a better movie - more entertaining although also loosely based on a true story. Peschi once again plays the psycho. Goodfellas is a TRUE story; Casino just loosely based on one, so they can fiddle with the facts for more entertainment. The best gangster movies I've seen personally are
A Bronx Tale
Donnie Brasco
Untouchables is pretty good too. So is Road to Perdition, yes, and of course Godfather 1-3. But Brasco is great, and based on a true story, which makes you bite your fingernails even more. Bronx tale is awesome (though I don't think true), and has DeNiro in minor role as the father. I'm sure there are many ganster movies I've missed though.
PS. Oh yeah, great directing in Goodfellas with the scene entering the restaurant. That took some serious choreography.