How to interpret MUMmer plots? When do you know that your alignment was a good one?

Question

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I have been running some alignments using MUMmer between the contigs.fa that I have received from Velvet after genome assembly with the reference genome, and I have been trying to find out which of the assemblies I have run were the best ones. For instance, for one of them I received this plot, does this look like a good or terrible alignment?