I absolutely love the chemistry Capaldi and Mackie have already.
I enjoy that she's not the usual companion and a bit slower on grasping what was considered 'mundane' last season. Bigger on the inside, two hearts, that stuff. We got that little jumping on 'it's bigger on the inside' scene last year, so this is a pleasant change of character.
I'm also not as opposed to Forest or this episode as some are (that I recall. I'm sure my post history might contradict that). There are some episodes of the RTD era will probably always be my 'lowest point' so as long as the plot is above that, I'm fine with it. Then again, there's about ten years between those points and production has obviously changed in between.
I too, noticed that the insides of the ship were too big (and shiny) to be a set and am wondering where they filmed those parts as well (though the engine room itself is a set again, I suppose). The intersection from the Valencia museum / the city to 'the ship' could be shot on a set, whereas those two main parts seem to be shot on locations. One we know, and the other we don't. Probably just a factory or refinery or something, but still. Not that 'shooting on location' is limited on this show though.
edit: to clarify, I actually thought this episode was solid.