They're a divisive topic, I'm peased with mine for looks alone and I really wasn't expecting much but I got a smoother idle. There's plenty of posts where people have fitted and not reported any change and I can't recall the thread but I'm sure someone said you could achieve the same thing by cleaning and tidying up existing grounds.
For the flip side of the coin h-dev have some findings in the blurb on their website that's worth reading in the interests of balance. http://h-dev.co.uk/product_info.php?products_id=25417
At worst they're a nice bit of engine dress for not much spend, if you can make one up that matches you're engine bay theme, I personally don't see a downside.