Extremism in anything is bad and horrible. So, no matter how beautiful a thing is, once extremism comes into the equation it becomes ugly. So, extremism even on national grounds is wrong.
It depends on what you mean by extreme-nationalism. If you mean that you are for your country no matter what and behind it for good or bad then I believe it is a great thing.
I would call myself an extreme nationalist in that definition of the term. The beauty of the world today is that if you do not like where you are you can actually leave and go to a place that you do like. There is no end to the type of country and it's governance in our world as it has become so diverse while shrinking at the same time. If you are a citizen of country and you denounce the country of your citizenry, then, as a nationalist, I think you should leave and I wouldn't want to hear anything negative you have to say about it. Period. It is the type of mentality that says, "Don't come in my house and tell me you hate it or you are going to hear about it!"
On the other hand, there is a form of nationalism that is more racist than nationalist. I think that is not only bad but terrible. It's basic tenet is that "if you don't look like me or act like me I hate you". This is wrong. This isn't nationalism but racism trying to pretend it's nationalism. Not only is this terrible for moral reasons it is detrimental to a country.
The reason why the US has enjoyed so much wealth over it's life is because its people love the country (nationalists) and we are pro legal immigration. We want others to come here (legally) and enjoy the love for country with us as well as offer their entrepreneurial abilities to our economy so that while they share in the work done before them (ie. free-market, freedom, infrastructure, society, etc) they can add their part so that more can enjoy the future.
So, I think it's a good thing. A pride and love for ones country on a large scale will always be a commonality that large groups of people can share and strengthen under.