Firstly, if you've seen earlier Gundams (MSG, Zeta G, ZZ) and didn't like the plot in any of them, you probably won't like that of Gundam 00, either. You can still enjoy it if you keep your expectations low, though.
In the first few episodes, it's suggested that Celestial Being's actions are based on an ulterior motive, and many characters have hidden pasts that are revealed little by little -- and so, based on previous experience, you might expect (hope) Gundam 00 to have a "strong" plot. As it turns out, it becomes much weaker as the series progresses. The events become more and more isolated (they either don't contribute to the big picture or they change it in an illogical way) and mainly seem to serve the purpose of suddenly making the action scenes more 'exciting' or suddenly creating a sense of 'tragedy'. Many plot details end up being irrelevant to the big picture, and on the other hand many plot twists are brought about by a deus ex machina. The final episodes are almost pure action, and the plot becomes a complete mess as it hurriedly tries to tie up loose ends (or again, make the action more 'exciting' or the plot more 'tragic').
The cast is varied and starts out pretty interesting, but most of it hardly receives any development (at least in the first season). Worse yet, some important (-ish?) characters die before serving much of a purpose.
In summary, as long as you watch it for the action or because you like some character, you should enjoy it --- if you hold high expectations for the story, you'll probably end up getting disappointed like I did.
