An ex-MP has pronounced that St Albans would have elected a Lib Dem MP a the last election, if it had been under the Alternative Vote system.
This is possibly true, and possibly not true. One can have no idea what was in the heads of local voters under first-past-the-post, especially in a constituency like St Albans where such a huge number of people voted tactically for the Lib Dems (ironically, "to keep the Tories out").
AV is an improvement on the current system because it gives more people a feeling that they helped to elect the MP in their area - at least 50% of the local electorate will have expressed some kind of approval for each winning MP.
But the AV system won't change the end result very much, as the three main parties will by and large continue to win where they are already winning. The system is not "proportional" because the first preferences of votes are not properly reflected in the make up of the House of Commons. At least, not any more properly than they are now.
So AV is a small step in the right direction. Which is of course why most MPs will be campaigning against it.