Many people claim that anarchists are just idealists with an unworkable utopian ideology. This is most likely due to the fact most people equate anarchy with anarchocommunism, which is a utopian ideal, and an oxymoron to boot. For that, I blame Marx, and because of its inherent contradictions I dismiss anarchocommunism as a pipe dream held by the insanely delusional. As such, I will be focusing this discussion in relation to the one logically workable form of anarchy that I've found - anarchocapitalism (or voluntaryism).
Utopias & why they are not anarchy
First off, a utopian society has a single solution for any problem, and one which is perfectly suited for everyone. A utopian society would also have no disease, no crime, no conflicts, and no economic failures. It would be a place where everyone gets what they want and need, without having to do even more to get it. If you have listened to political rhetoric during this year's elections, you will probably hear at least one candidate promising one of the above conditions, and that is because government constantly tries to portray itself as a provider of utopian solutions (when in fact they are anything but).
What Anarchocapitalism Suggests
Anarchocapitalism doesn't promise a single solution for anything, nor does it claim to have all the answers (yet). It doesn't even try to claim it can solve every issue outright via a single method. What it claims is that everything should and can be done with voluntary consent for those who want it, and as such every currently ethical solution must be available as a choice. If they do not consent, then they do not participate in that exchange. If they want a different condition for the exchange, they can find another provider, negotiate for it, or begin providing that service themselves.
It doesn't assume people will be inherently altruistic. It assumes they will be focused on their self interests and greed, and it uses that greed to correct for everyone else's greed. This may sound illogical, but it actually isn't. If no one person's interests are given priority over anyone else's (like they are when the state is in charge), then everyone's self interests has equal weight in that society.
It would be like a tug of war happening in all directions, using an untearable sheet instead of a rope. If no one entity has the weight of the state supporting it (like corporations have today), then the sheet is pulled tight all the time, but won't move in any particular direction, or if it does, it won't be for very long or far. With the state, corporations can drag the sheet all over the place as they see fit (like they do today) , but without that protectionism provided by the state, there is little they can do to force competition down, and keep the sheet moving their way.
*woah, hang on... Isn't there a flaw there? *
People might say "but what about collusion?" to which I say "where is the benefit in a free society?" Companies that collude to control a market eventually get found out, and without the added barriers to entry created by government, new businesses can arise to compete with those colluding entities.
It is not in a weaker company's interest to collude with a big fish, or a group of them, unless it is impossible for the weaker company to acquire the strong entity's customers. As all the smaller company needs to do is offer the similar products for less to gain the bigger entity's customers, why would small businesses collude? It can't grow as quickly if it does, and therfore can't gain investment as easily, making it harder to do more business.
Other reasons why anarchism isn't a utopia
Anarchocapitalism requires people to take complete responsibility for their actions, and requires people to work to live. That's a lot of hard work, and as a result can hardly be considered utopian. With full liberty, however, individual people can decide if they want to pool resources to share in a publicly funded service for those who contributed, or they can find another provider they feel is more reasonable in price or quality. No one will be perfectly happy, but everyone will have every option to try and increase their happiness.