I've been voting LibDem since I was 18: I mainly started because we had a talk from them at school where they talked about Proportional Representation (PR), and that sounded like a fundamentally fairer system than what we currently use. Since then, I've read up on their other policies (which I agree with), but I've started to have doubts about PR, mainly because it allowed the BNP to get seats in council elections. I then saw some billboard posters from the No to AV group, which said that AV will cost a lot of money (£250 million), and that we have better things to spend that money on, e.g. hospitals: that sounded like a valid point.
I've now done some reading on the issue, to understand it better. Since this is politics, I think that both groups have made some claims that border on fiction. However, the "No to AV" group are clearly complete dingbats, so I'll be voting Yes in the referendum.
Here's a short clip from Pirates of the Caribbean: At World's End which illustrates our current system:
(The actual voting starts at 3:59) There are 9 people voting, and the winner got 2 votes. That was more than any other candidate (since they all got 1 vote each), but hardly a majority, since 7 out of 9 people voted against her. Under AV, the winner would need to get 5 votes out of 9.
It's important to understand that AV isn't PR. So, whether you think that PR is good or bad is irrelevant, because that's not one of the available options. Right now, we just have two choices: FPTP and AV. Looking on the BBC website, here's one of the featured comments:
"I will not be voting yes in the referendum because I believe AV is a poor alternative to what was originally promised by the Libdems: proportional representation, but don't mistake my no vote as being one of support for the FPTP system."
(#118 from "StumpyPumpy")
Here's the thing: if you vote no, you are supporting FPTP by definition! If you genuinely believe that it's a better system than AV then that's fair enough, but if you'd prefer AV then you're just sabotaging yourself.
As I see it, the main problem with FPTP is that it encourages "tactical voting". For instance, I remember John Cleese doing a LibDem party political broadcast in 1997 (video). Key quote: "The polls show that half of you, 1 of every 2 people watching this now, would vote for the Liberal Democrats if you thought we could win. [...] Because of course that means that if you did vote for us, we would win."
I don't know how accurate those polls were, but there's certainly a perception that your vote is wasted unless you vote for a major party. (The definition of "major" may vary between constituencies.) The Simpsons really nailed it: "Go ahead, throw your vote away!"
This diagram illustrates the problem:
In that example, if the people who voted for "The Red Lion" had voted for "The Castle" instead then they would have been able to go to the pub after all. So, you need to predict how other people will vote before you cast your vote; meanwhile, those other people are trying to predict how you will vote! AV makes it a lot simpler: you just choose what you would most like, and put that as your top choice.
If one candidate (party) gets a majority of the vote then nothing will change under AV. For instance, I live in South Croydon, and the Conservative candidate got 50.9% of the vote in 2010. So, he'd win the seat under FPTP or AV. In fact, he has held this seat since 1992 (19 years), and his predecessor (also Conservative) had it from 1974-1992 (18 years). So, I'm not convinced by the Yes to AV group when they say that AV will eliminate a job for life. On the other hand, I don't see that as a problem: if someone is good at their job, and popular, it's fair enough for them to keep getting elected.
In practical terms, I don't think that AV is complicated. Ranking parties in order of preference is simple enough to do, and if you'd prefer to just choose one then that's still an option. It's more work for the people who have to process the voting forms, but that won't be an issue for you unless you're a volunteer. It may also take a bit longer to get the election results (if the process involves several "rounds"), but I don't see that as a problem. There's no real need to stay up late waiting for election results when the new party won't take over for several months, so just be patient!
Similarly, the system won't be anywhere near as expensive as the "No to AV" group claim. There's a breakdown of their estimate here. In particular, there's no need for new computer systems: it's perfectly possible to do it all on paper, like we do now. In fact, it may be better to do it that way, to avoid any suggestion of fraud. Bruce Schneier has spoken out against voting machines in the USA, e.g. here and here. So, that completely undermines the billboard posters, and I think it's sufficient evidence to distrust everything else they say.
The slogan on the front of the "No to AV" leaflet is "Keep One Person, One Vote". pozorvlak has posted a good rebuttal to this: Why "One Voter, One Vote" is FUD. When I first saw his post, I actually thought it was a strawman argument; I couldn't believe that anyone would seriously suggest it. I was quite shocked to see that William Hague had in fact argued this: either he doesn't understand how AV works (in which case he shouldn't be in politics) or he does understand it and he's deliberately lying. So, sorry for doubting you, pozorvlak... Anyway, it's a bit like Big Brother, where candidates keep getting eliminated until someone gets a majority. Everyone gets one vote per round, and the only difference is that some people will vote for the same candidate each time while other people shift their vote to a different candidate after their first choice is eliminated.
In closing, I'll leave you with this comic: Parma violets, Smarties, or dogshit?