The "Alternative Vote" is clearly a huge improvement on the "First Past The Post" system currently used to elect MPs in the UK, mainly (in my opinion) because it means I can simply write down my order of preference on the ballot paper, rather than having to second-guess how everyone else will vote and deciding to vote for my second-choice candidate in order to keep out my third-choice candidate, only to find that my first-choice candidate fails to win by one vote.
So I'll be voting "Yes" in the referendum on 5 May 2011.
But it doesn't exactly help the cause when the main proponents push inaccurate claims such as this:
All MPs would have the support of a majority of their constituents
Choosing the Alternative Vote means when a winner crosses that finish line on Election Day they’ll have to bring the majority of voters with them.
Phil Walker does a nice job of explaining all this (it's quite simple really) and Wikipedia has a real example, in which the winner of a mayoral election in the US, held using the AV, had the support of 48.6% of the voters.