Proportional representation is all very well but, in a milieu of a government with only a slim majority against a strong opposition, you effectively hand a casting vote to the minority party in almost all split-decision situations.
This means that the public needs to be *extremely well versed* in how UKIP, SNP, Plaid and the Ulster parties stand on aspects of the sitting government's manifesto program.
Paying attention to all the parties which you don't actually vote for is going to be tiresome, as well as counter-intuitive.
Meanwhile fans of Borgen will recall the required horse-trading (ministerial positions in return for coalition support) required to achieve a majority, in a state with 6 or 7 parties. Adoption of PR will surely encourage new ones to set themselves up, for a piece of the action.
Our own coalition experience, "Tory policies but with the handbrake on" was, erm, educational but my abiding memory was that I had half a mind to vote Lib-Dem because I'd never tried them yet. I didn't follow through but I never envisaged them coalescing with the Tories, having distant memory of a Lib-Lab pact, from the 70s. I now regard them as total sell-outs and cannot, in all conscience, trust them with my vote in future. If I want Tory policy, I'll vote Tory.