Completely depends on what dialect they are speaking. Most Norwegian dialects are like hearing them speak Swedish but with more "e" instead of "a", so yeah, it's pretty easy. But of course there are some that's pretty much impossible to understand, but those mostly speak in local dialects. Sometimes I get surprised on how terrible some Swedes are at Norwegian, but most people understand it.
Norway got a lot of different ways of writing (nynorsk and gammelnorsk (new and old norwegian) are the most common ones) , but most of it is understandable.
Swedes, Norwegians and Danes basically can speak to eachother, but there are always some that are hard to understand. Danish can give you headaches, it's just too messy to listen to ^^ (imagine someone that speaks Swedish/Norwegian with some mud in their throat, that basically sums up Danish
)
Icelandic is similar to the other languages, but we can't speak with them just like that. They are more like a native language that noone understands anymore.
No need to mention Finnish, that's a completely different language (some words are still the same in both Swedish and Finnish though).
