The argument tends to go something like "First Past The Post (FPTP) is an old, unfair system where people don't get the result that they voted for. PR is better because it is fair", there are a number of significant holes in this argument.
Firstly many people talk about PR as if it a voting system, it is not. It is the generic name for a range of voting systems which include three major systems, Party list system in a multi-member constituency, Additional-member system, mixed-member system, Single transferable vote in a multi-member constituency and there are number of variations of system within these including: condorcet methods, Copeland's method, Kemeny–Young method, Nanson's method, Schulze method, Bucklin voting, Coombs' method, Instant-runoff (Alternative Vote), Contingent vote, Borda count, D'Hondt method, Sainte-Laguë method, Hare quota, Droop quota, Imperiali quota, CPO-STV, Schulze STV, etc. etc. I don't pretend to know or understand half of these and I strongly doubt that most British voters do either.
When it comes to fairness it is difficult to define. One idea of "fair" is that does the political distribution of MPs reflect the political distribution of votes? Another is that the actions of the government reflect the manifesto commitments in proportion to its political composition.
I call the first test the "Who I Wanted" (WIW) test and the second the "What They Promised" (WTP) test. FPTP scores lower on the WIW test than PR systems and PR scores lower on the WTP test because minority parties tend to have disproportionate power when coalition talks take place, often known a the "tail wagging the dog". This what happened in the London Assembly when Ken Livingstone relied on the Greens to get his budget through, you might be happy with the Greens getting more power than their votes would dictate but would you be so happy if it were the BNP that held the balance of power? Neither system is completely "fair" they are just unfair in different ways.
There are also some big issues around accountability, many PR systems shift the power over candidate selection away from local parties members and towards party HQs, as they are based on party lists rather than constituency members. If there is a particular MP who you feel is acting bady over expenses, for example, how do you punish that particular individual? If the bulk of a party's MPs were excellent it wouldn't be fair to vote against that party. And what would you do if all parties had one or two bad apples?
With PR systems it is often impossible to punish a government for failing to implement their manifesto, they always have the defence that they didn't have a clear majority to do so, "blame the coalition partners not us" they say, which is usually what voters do. This leads to lots of elections and very few changes of government. Far from "fair".
If FPTP was lined up with all its pros and cons against the various PR systems with their pros and cons I suspect that PFTP would win out. We may well get the chance to see.