Depends what you want to do- a 24" bike will be good for getting around on, but the geometry is going to be very different from a 20". The only 24" bike that will really give you the same 'hop' and handling as a 20" is probably a Sunday Model-C, which has a *much* different geometry than the typical 24" frame (the Model-C has a higher bottom bracket, shorter chainstay, etc.) but those are expensive.
Typical 24" BMX bikes have 'race' geometry, which means it is designed for going very fast and being very stable. They usually will have a low bottom bracket and long chainstays, making it difficult to pull up the front end. It is not conducive to bunny-hops or freestyle.