Really good, but get a pair if you can.
These Praddels are really useful, and stow away so easily that you can have it with you every time you go out. It takes up less space than a small bailer.
However, if you are using it in a single-handed dinghy, and you are not a small child in an Optimist,, then you need to know that whilst definitely better than nothing, they are not that great, as to use one you have to be right over on one side of the boat, thus tipping it over and causing it to self-steer to the other side even more than it would if you were say just using one oar but sitting in the middle. In my little dinghy the effect is so pronounced, and I have to counter-steer so much with the rudder (pushing the tiller away) that it really slows the boat down a lot.
So these are definitely, definitely better than nothing in a single-hander, but it is in a double-hander that they really come into their own and work well, as then you can balance the boat to keep it level. And with a Praddel each the boat really scoots along well. You can even ship the rudder, lash the tiller, or lift the blade to stop it annoying you if you each have a Praddel. And a pair of Praddels nests together and takes up no more space than a single one.