Raising Steam was mostly OK. The plot became too predictable and formulaic mid-book. But there was one bit that made my day, and that was Albrecht Albrechtson's speech to Ardent. The way Pratchett depicted a conservative politician that was actually someone I could respect and even admire - which rarely if ever happens in real life. One who had good reasons to stick with the Old Ways, and who came to realize those reasons no longer applied (or at least not as strongly as before) and was open to change, as long as change was supported by reasonable justification.
One of the weakest points: With its history in many past books, when so many Watchpersons are present, it is hard to find any crime much of a challenge. Which makes it almost impossible to write any kind of books featuring Vimes or the Watch heavily believably suspenseful. Vimes has become another Vetinari - can add flavor if used sparingly, anything beyond that is overdone.
Re: Vetinari's characterization: To people who complained earlier in this thread: This is not the first time Vetinari openly mentions being a tyrant. He did it in Making Money too. I don't think he is ever subtle with Moist, come to think of it. The chatty Vetinari has been around since at least Unseen Academicals.
As for at which point Charlie became involved - somewhere around page 200, after Moist leaves the Oblong Office, Vetinari asks Drumknott how Charlie's clowning business was doing. I think that was when Vetinari first raised the possibility of employing Charlie as his 'replacement'. Between that moment and the beginning of the train ride, there are three scenes in which Vetinari appears, and in at least one of them Drumknott isn't present, so that limits the possibilities for scenes where we haven't seen the real Vetinari. I do think Vetinari must have taken a few forays as Stoker Blake before the big train ride to Uberwald because he already had the reputation, so Charlie may have had a few visits to the Oblong Office during the weeks leading to the train ride, though I doubt we saw more than one of them.