Viewed from this side of the Atlantic, Ben Stiller's career is a rather strange one. One minute he's a writer/director/actor, then The Cable Guy bombs, he stars in There's Something About Mary, follows it up a couple of years later with Meet the Parents and then proceeds to make a lot of duds, but has enough hits to remain in demand. Night at the Museum was phenomenally successful from a purely business standpoint and as such a sequel was a no-brainer.
Here Stiller reprises his role as Larry Daley, who since the first film has stopped being a security guard at the Museum of Natural History (where the exhibits come to life after dark due to an Egyptian tablet) and become a successful inventor/entrepeneur. Visiting the museum he discovers all his old "friends" are being packed off to the archives in the Smithsonian and there's nothing he can do to stop it. He then gets drawn into saving his friends when the tablet gets taken with them, thus bringing the world's largest museum to life.
Stiller is fine, the film opens with a series of cameos (Ricky Gervais, Jonah Hill) which fall utterly flat and it takes far too long to execute the plot set up. When Amy Adams arrives playing Amelia Earhart it is a breath of fresh air, but still not particularly amusing. The person who provides the most entertainment is easily Hank Azaria who absolutely comes to play doing his best Boris Karloff impression for the suitably maniacal baddie. The supporting cast are sorely underused, in particular the hilarious Alain Chabat who has a nothing role as Napoleon and is obviously just picking up the pay-cheque. This is nobody's idea of an actors' film though.
It's all very watchable, the special effects are first rate and once it gets going the film does move along. The problem is it's not consistently funny enough and the love story is utterly limp as amazingly (considering the tightness of Adams' trousers) Stiller is unable to generate any kind of chemistry between them. In films like this a certain amount of good will is almost demanded and it is fine for what it is, but the end in particular is a complete cop out. A decent way to shut off your brain for an hour or two, but nothing revelatory here.
Oh, boo you for not liking the Jonah Hill cameo!
But, yes...Gervais was pathetic and doing his usual routine. Yawn. And you're right again - the comedy was inconsistent and Adams, despite a cracking arse 
, killer last line delivery in the role of Amelia (I had a tear, GODDAMMIT!) and ever present adorability, gets completely outshone by Azaria. I find myself endlessly quoting his lines - "Is that you breathing? Because I can't hear myself think! There's too much going on here; you're asthmatic, you're a robot. And why the cape? Are we going to the opera? I don't think so." BAHAHAHAHA. The 'crossing the line' killed me too. I love the guy.
Bill Hader's "We're American. We don't plan, we DO!" line is all kinds of excellent as well.
Anyway, it manages to retain a good level of watchability and it's the first time Stiller hasn't bugged me. So...it gets points for that too. I had low expectations, but I found myself won over by its charm. Not great by any stretch, just utter silliness for an hour or so. Can't object to that.