I have tested this stuff and found it to be as good as, if not better than Harpics Power Plus Toilet Cleaner!
I spotted this the other day in the Isle of Wight store and thought I would buy and test a bottle before stocking up, it actually seems to work better than Harpics toilet cleaner, it did a better job and in less time. I know because my toilets get quite stained with limescale, I'm a fan of 'if it's yellow let it mellow, if its brown flush it down'. This stuff saves me money on the water bill ;D
I'd like to point out Harpics is also on offer for £1.20, usually £1.60, but I posted this one because I find it better and it's almost 50% cheaper.