Expensive for a piece of moulded plastic, I suppose, but you're also paying for the design and engineering and quality. It fits perfectly, even after hundreds of on-offs, and does what it claims to do, i.e. give you a soft light source. It is a rigid piece of plastic that fits around the flash head precisely, and from my experience will last a lot of use.
One thing: it will not give you modelling or shadows (using bouce flash will be better in this respect, but that may have colour temperature pay-offs), so the results may be rather flat unless you use a slave flash. I haven't found the reduction in flash power (using a Nikon SB 600 with TTL on a D90) a problem: exposures were still accurate, and any extra flash power simply meant I had to change the rechargeable batteries a bit earlier, though not so you'd notice.