I saw Dawn Treader this evening.
I enjoyed it. I might even venture that I enjoyed it more than a little. The nature of the source material let Dawn Treader be its own thing, rather than the Lord of the Rings-lite that the previous two films tried to be. The story did feel a bit slight, but that's an artifact of the books. The ending, on the shores of Aslan's Country, was moving; I'll even admit to shedding a few tears.
To answer the question posed at the start of this thread, I think the film series could carry on, easily, with Eustace; Will Poulter was certainly compelling and charismatic enough. However, the ending also brings a certain amount of closure (since Lucy and Edmund are now cut off from Narnia), and if there's not a Silver Chair, the series will feel somewhat complete.