I've tried them on absolutely everything! Bunny Brunch, Pets At Home musli, Super Excel, Science Selective, Russel Rabbit...
For 2 and a half years I gave them Bunny Brunch, and throughout that time they had diarrheoa every day, and were quiet and inactive. I'd never had rabbits before and didn't realise it was unusual - I thought the diarrheoa was just the soft droppings they're meant to do. When they were about 2 and a half they just seemed to stop eating the Bunny Brunch, picking out only the vegetables I provided. That was when I started experimenting with different diets. I eventually found 'Dr Susan Brown's hay and vegetable diet' In 'The really useful bunny guide' by Carolina James (it is REALLY useful - I reccommend it for all bunny owners). This was just hay, vegetables, a small amount of fruit and the occasional wheat product. Since then they've never looked back, (although my pocket has - wistfully!) They've become full of energy and are generally very healthy and happy, just as buns should be.