I used to support instant runoff voting. In fact, I voted to change the UK voting method to instant runoff in the 2011 UK Referendum.

However, I've become less enamoured with it. People don't understand it. It's used in London for the mayoral election, and people just get confused - they'll tell me they are putting the same party as their first AND second choice.

In fact, judging from the mayoral election, it wouldn't change much - the two main parties still end up getting the lion share of the first round of voting, which implies that the vast majority of people are loyal to those parties (a bit like football teams) and aren't voting for them out of tactical necessity.

I'd still go for instant runoff in a heartbeat, but it's not the panacea I thought it was.