Green River knives are great for the price; they easily take a keen edge and slice very well. But note that they are pretty basic in materials and construction. My only real gripe with them is that most of their designs do not accommodate a guard to keep your hand from slipping forward onto the edge. This is not uncommon, there are many traditional knife patterns that are well loved and still popular that do not have a guard, such as the puukko. Not difficult to learn how to use them properly, but it is something to think about.
The blades are also very thin which makes them excellent for slicing but not advisable if you are the type to use a knife for even light prying.
Finally, Green River knives are made from pretty basic carbon steel, meaning the blades will rust easily when wet or exposed to acid (like juices from meat, fruit, and vegetables). Again, that's normal for traditional knives and pretty much all knives made before the 1920s, but it requires a bit of care and maintenance (mainly keeping the blade dry and clean and occasionally wiping down with mineral oil).
I like Green River knives because they are nostalgic, maintenance is not too burdensome to me, and because they just work. That said, knife designs and materials have steadily improved over the nearly 200 years since Green River knives started up.