First off, 'A Rising Thunder' is not bad. It's not great and I suspect it'll be a much better read in hindsight, but it is somewhat awkward, as it spends so much time trying to tie together all of these separate strands and people together rather than just getting on with it. In turn, this is due to the spectacular quantity of arc-welding that Weber's having to do to bring the originally planned storyline forward by about forty or so years.
The result is a bridging volume, much like the other reviewers have said. It has no real beginning, starting as it does somewhere nebulous in the beginning of the previous book, and it has no real end due to publishing constraints (A Rising Thunder and Shadow Of Freedom were intended to be all one book, but then got split because it all got too long). Hence why I'm reckoning it'll be much better in hindsight, i.e. when the rest of the series is here so we can just merrily read on through it to the next book.
All that being said, the book does have some stand out moments (the Beowulfan speeches before the Chamber of Stars come forcibly to mind, both of which rival Queen Elizabeth's speech at the end of the last book), and fills in a LOT of plot and setting.
My advice to prospective purchasers, if you haven't bought this already, wait for the paperback and the next one to have come out, then read them all at once. (Also, if you were as psyched as I initially was for the second battle of Manticore, remember that the agressors are Solarians and calm down some...)