I think the main problem with SQ's Premium Economy offering is that they focus on useless gimmicks (such as unlimited Champagne) to justify the 2x price difference whereas a carrier like BA gives something more more meaningful (100% increase in baggage allowance).

I flew in the cabin late last year and I can't say I was impressed (Review and pictures here on my blog if you're interested!) and I'd rather save the money by flying normal economy or just save a little more and fly business!