I read them all one after the other last year, and was blown away by what a difference this made! Up until then, I had always read them out of order. Of course, I agree the themes sometimes repeat, but Enid always manages to include enough twists and turns that they still seem fresh, at least in my opinion.
I found it very interesting, reading them in order, how sometimes Enid would take a phrase or an idea used in one book and expand it in the next, sometimes into a full plot. I can't, unfortunately, think of an instance right now, but I noticed this several times when reading them in order.
There are of course a few times she reuses plots and I think the second use of the same plot is never so effective. One big example of this is 'Five Are Together Again' which is a much weaker version of the plot of 'The Rilloby Fair Mystery'. But most of the time the recurring plots still seem slightly different and fresh because of various added elements.