Mine is on its own in the 50 gallon which is cycling. So it is hard to judge its performance as of now.
A U4 would be to big for a 24 gal, it would take up a quarter of the tank to be honest. But it would definately be capable of being efficient in that tank.
All's I can praise it about it has good/powerful but adjustable oxygenation system. It is also well built and easy to use.
Im afraid thats all I can give at the minute, because it is not old. So in answer to your question: Yes, it does work how I thought it would and Yes, I think a 'U' would benefit your tank.
James.