Did a little experiment with this yesterday. There are 7 of us in our house, so we have a lot of washing and seeing as there are 5 aged 9 or under, that washing is usually caked in mud!! We usually use Ecover but have always seen this as not being the most fantastic option, just the better of a bad lot.
Anyway, tried two loads of dark colours yesterday - one with just water and one with an old tennis ball that has lost it's bounce and gone bald (bit like DH

) The addition of the tennis ball did seem to make a difference and did seem to clean reasonably well - in both loads there were some items that didn't clean as well as they do with Ecover, but I think that in the future I shall use the tennis ball for clothes that are in need of a wash, rather than filthy - IYKWIM
Lighter colours at the moment will still need Ecover, but I'm now going to start using less each time until I find the bare minimum that I need.