The arguments I have seen used against AV are incredibly disingenuous and insulting.
The silliest and most dishonest argument has got to be "someone can be elected on x little percentage of the vote. How is that fair". Actually, that's more likely to happen with First Past the Post: currently, if you vote in a constituency which has got one Labour candidate, and a lot of rightwing candidates, and the rightwing candidates split the right-wing vote, the Labour candidate could get in with a small percentage because the left-wingers could have all voted to keep out the right-wingers. In France before Sarkozy, a split leftwing vote kicked Socialist candidate Lionel Jospin out of the election, meaning a final round between Chirac (conservative) and LePen (racist fascist). Shocked and humbled left-wingers then had to vote for Chirac, but did so with clothespins on their nose. This is the sort of surprise shock you can get from an uncompromising choose-one-or-the-other voting system. Any kind of voting system which allows you to rank candidates means you can send exactly the message you want, not let someone crazy in by accident.
'Nick Clegg is unpopular, so he wants you to vote AV to save his party, which has been a turncoat on a lot of its manifesto pledges during its time in coalition with the Conservatives'? Well, if Nick Clegg and the LibDems are so unpopular, AV will actually ensure you can make sure your vote never goes to a LibDem (because you say if Green don't get it, then Labour, then Monster Raving Loony Party, etc. Easy to exclude anyone you dislike).
The Times had an editorial about how 'anyone who hears about AV initially likes it, but then the more you explain it to them, the more confused they become and the more they dislike it.' How vague is that? Who did the explaining? Who are these compliant "people" whose little heads were "confused" by something the columnist failed to make plain? FFS, if you can't explain what's wrong with the system, just that "people don't like it", what sort of journalists are you? Crap ones. Oh, and a very important point these Times writers didn't make was that the Times is a Murdoch paper. Murdoch as a business entity is well-known for political donations, political pressure, the Sun's boasting that "It's the Sun Wot Won it" and so on. Is Murdoch going to be keen on any kind of new politics which means he has to donate more, to more political parties, instead of exerting influence fairly easily in the current cosy system of 2+ parties? 
The behaviour of those damning AV has been pretty disgusting, as well. The Conservatives agreed to a referendum, as one of the costs of coalition. So they agreed to whatever they had to, to get the LibDems on board, or they would have been powerless, and now they are turning on the LibDems with personal attacks and dishonest campaigning. Pretty dishonourable. Why would they behave like that? Openly, too!
As for the supposed cost of converting to AV, that's scaremongering, and again, pretty vague.
I am just really insulted that this anti-AV campaign has been waged so nastily, dishonestly and with a more or less open message: fuck off and shut up and leave us to it. I don't like being spoken to like that, still less by people who ought to be dependent on my vote.
I am definitely voting for AV. It may not be perfect, but there is no way I will let anyone think I agree with the anti campaign.