I use Wet and Forget and really recommend it. I only need to use it once a year and each year it seems to work more quickly. The Amazon deal is a good price, but I've got 7 litres ready and waiting for the next few years so I won't be buying it.
I have also used spray and leave from IW. It's comparable to Wet and Forget with regards to its effectiveness. To be honest, when I need some I get the option that's the cheapest. My last purchase was from Costco and it was a little cheaper than the Amazon price as it was on offer.
