This Thursday, the UK is holding a referendum on voting. Specifically, voters are being asked whether they want to stick with the status quo--"first past the post" (FPTP) or "plurality" voting--or to try something new. The new method they are considering goes by the name "the alternative vote" (AV), but it's known in the US as "instant runoff voting" (IRV) or "ranked choice voting" (RCV), and unless you're new here you've heard me heap criticism upon it. For this article, I'll be using the UK terms.
Current polls show the measure is teetering towards failure, which if fine from my perspective, but both the "Yes" and "No" campaigns have been flying some pretty ridiculous propaganda, so let's set the record straight. AV will not cause democracy to crash and burn. They've used it for a hundred years in Australia, and it hasn't turned them into an arid wasteland of venomous monsters (well, no more so than it already was.) Nor will it create a utopia of perfect democracy. Again, Australia has used it for a hundred years, and everyone complains about the government just as much as they do in any other free country in the world.
In practice, AV gives about the same results as FPTP. I've seen a claim that over 100 seats would have been allocated differently, but that's based on counting any seat where the winner had less than 50% of the votes as a potential switch; the actual number, based on reasonable assumptions about voter's second- and later-choices, is 3, or 1%.
Furthermore, with perfectly-tactical voters, AV always gives exactly the same results as FPTP: It is only beneficial to the degree that voters will choose honesty over tactics, and despite the "Yes" campaign's claims, AV voters still can benefit from, and so will be incentivized to practice, tactical, rather than honest, voting. This is trivial to show, especially in the situation the UK finds itself in now, with three strong parties that have no natural and obvious split among them as a basis to form a coalition. While it is true that AV makes it more-difficult for a small party to act as a spoiler--which is the kernel of truth that the "Yes" campaign has built this fabrication around--it has the same susceptibility to spoilers as FPTP when there are three or more strong parties, and it's fear of spoilers that drives tactical voting. (AV can still result in hung parliaments too, for which I again point to Australia, who saw its two largest blocs walk out with 48% of seats each in their 2010 election.)
While AV does, in a limited way, shore-up this one weakness in FPTP, it also introduce its own special failure modes, including non-monotonicity, participation failure, and other paradoxes of voting which cannot occur under FPTP, but which are quite common under AV. The improvements are small, and then almost entirely countered by these new failures. Failures which commonly are used as the basis for repeal campaigns by voters who feel they've been sold a false bill of election-reforming goods. (Expect the repeal movement to peak after two or three elections have passed under AV.)
But even if we follow the naive notion that every voter would choose honesty, and that the marginal improvements are worth the new paradoxes (and that the inevitable repeal movement can be fought back), then AV still comes at a steep cost. Ballot-spoilage rates are four to seven times higher with AV. Not, as the "No" campaign has said, because voters can't understand the process, but simply because there are so many ways to make a ballot-invalidating error when filling out such a ballot. Perhaps related, the counting process is notably more expensive; not astronomically so, as the "No" campaign has claimed, but it will require either the purchase and maintenance of more complex, and therefore more costly, voting machines, or a more time-consuming, and therefore more costly, amount of hand-counting.
The UK does have a few points in its favor that would make AV less of a wasted effort than it would be, for instance, in the US. The relative lack of effective political polling in the UK leaves voters with less information with which to make informed tactical voting decisions, so "honesty" may be a somewhat more-likely default than it would in the US. And since UK voters do not directly choose the head of government, a unitary office, there is less of central figure for voters and parties to rally, and become divisive, around. Both of these aspects increase the likely amount of improvement in outcomes due to AV, from "minuscule" to "tiny". However even these mitigating trends are on the wane, with political polling catching on and party heads taking a more active role as the faces of their parties.
Finally, there is the notion that a vote on this (non-)reform sends a message. The "Yes" campaign says that a yes vote sends the message that you want something to be done and this is the first step towards proportional representation, while the "No" campaign says that a no vote sends the message that this is not the reform you want and that you would rather have proportional representation. Uh huh. My advice is that your vote for or against AV should be based solely on the (lack of) merits of AV, irrespective of what better "sends the message" that you really want proportional representation (PR). The fundamental prerequisite for PR is multimember districts, and no government anywhere has ever enacted AV and then later been convinced to add multimember districts. The two reforms are essentially unconnected.
AV isn't worth the effort. I would encourage UK voters to vote no on AV, and to campaign for PR.