Parliamentary systems can work, but they require the right ingredients - e.g. an electoral system that allows for more than two parties, but is not based on absolute proportional representation.
In the UK, total lack of proportional representation, plus the lack of a real independent executive branch (Elizabeth is very well paid, but powerless!), means that Prime Ministers tend to become elected dictators on a minority of the vote, or if they're as idle as Cameron, their favourite Cabinet Ministers become the dictators. George Osborne, Michael Gove, Jeremy Hunt, and Ian Duncan-Smith at the moment. Lovely.
In some other countries, near-absolute proportional representation distorts democracy as much as the rigid first-past-the-post system in the UK, and means that small far-right or single-issue parties can hold a government hostage. Do you really want Mitt Romney getting to be president with less than 22% of the popular vote, obliged to appoint Pat Buchanan as his Foreign Secretary, and to allow Pat Robertson control of whether the government survives the next month? The Israelis are dealing with the equivalent of that. Worse, do you really want the KKK to take advantage of the economic crisis to get direct representation in your legislature? Ask the Greeks about Golden Dawn!
Some parliamentary systems seem to work very well, but I don't think that a parliamentary system is a panacea. And I do think that it's a good thing that in America, the legislators are elected separately from the president - I think the UK might benefit from that!