My experience of third party batteries is mixed.
Although they all work after a fashion, I find that some of them do not last nearly as long as original Canon Bp-511.
I have tried Uniross (dependable) Powermart (bought from *bay by another member, one works fine the other is only fit for testing) Blumax (ok so far but not used extensively yet).
I was shooting outside at night (maybe 4deg) for an extended time taking timed exposures and I managed to use all of my camera batteries with the original canon lasting the longest. If I am taking my camera away now I always put the Canon in (fully charged) and take a third party as back up only.
This is my experience using 10d,20d,30d.