Scotland, whose Parliament uses proportional representation via AMS/Additional Member System (one of the main drawbacks of which is that it takes an age to explain...), has had representatives from socialist/green parties win seats they wouldn't have had a chance in hell winning under FPTP ("First Past the Post"), which I think many people feel has lead to some pretty positive policies being debated and implemented which otherwise might not have happened.
AMS/PR has had the effect of forcing parties to work together much more in Scotland than has ever happened in the FPTP UK Parliament. I would dearly love for the UK Parliament to learn some positive lessons from the Scottish experience, but I doubt I'll see it in my lifetime.
I really like AMS - it retains the direct constituency link of a member for a region elected via FPTP, with the fairness of additional regional members selected proportionally resulting in a pretty proportionate outcome. The downside is that it creates two classes of member - those elected via FPTP, those elected via what AMS calls the 'list', and of course the complexity of giving people two votes instead of one that behave in different ways. The first vote is a classic FPTP, the second vote applies to the list. Your party's performance in the FPTP vote affects the "weight" of any list votes you receive, which is how the system remains proportional. As I said, it's a pain to explain, but one of the fairest systems I've experienced.