okay just used it ... here comes the review
Very very quickly does the job (I actually couldn't believe how fast it was!) In my 3ft tank it took less than 5 minutes to run it over the gravel and pick up the mess.
It did suck up some gravel though. I have 3 different sizes of gravel in my tank - one is a fine, almost sand, type. The next are soft round pebbles and because I couldn't get anymore of either recently I got a small bag of an in between size from p@h as the fish were pulling the plants up. I found it sucked up the finest of the gravel mostly but I don't think it's designed to be dug into the gravel like a syphon would be. It's designed to skim over the surface of the gravel to pick up bits of uneaten food and poop.
I have a valve for my pump that turns to flow up and down. Next time I may try it with that and see if that helps with the gravel issue.
To be honest, it's not a big issue. All I did was to tip the little bag out into a plastic cup and fill the cup with water. The poop floated to the surface and was easy then to drain off. I did this a couple of times then returned the gravel to the tank.
To sum up:
For £5.99 I think it's worth the money. I'm not sure what would happen if you have fine sand as a substrate though so maybe someone else can add a comment to that.
I certainly don't feel that I have wasted my money. I now have an item that I can use daily to keep my black gravel looking clean and nice and the plus side is that has to be better for my cories