I've still not been to the pictures much recently, but I did make an effort to see this, as I have a long and happy relationship with John Le Carre's work.
Basically: it's not at all bad, but falls down in some important ways. Firstly, I like the general look of the film: it's all a bit seedy and understated and the colours are unsaturated browns and greens. This does sometimes veer into a 70s pastiche perhaps (it looks more of its decade than the Alec Guinness version which was made in 1979). I also liked the portrayal of the Circus itself. Not sure where that huge building is that they filmed in but it has a great sense of place, of claustrophobia, and suspicion. It's a lot less cosy and more industrial than the cream-painted corridors in the old BBC version. There is more made of people listening in to phone calls, which sets the right atmosphere. The scene where Guillam has to steal a file from the library, and then gets hauled in front of the senior guys and roasted is very good. There are many other nice details, including the decision to make Percy Alleline Scottish (as he should be).
I feel one big error is the character of Ricky Tarr, who kicks the whole plot off. Ricky is a cocky little bastard who you wouldn't turn your back on, and was brilliantly played by Hywel Bennett in the 1979 version. The version here is all dewy-eyed sensitive boy who seems to have actually fallen for Irina the Russian trade delegate. As if Ricky would care, with all his other conquests.
A brief scene suggests to us that Peter Guillam is gay, which seems a pointless change to the book, in which we learn a bit about the nice female flautist he is currently seeing.
Many of Le Carre's lovely minor characters are gone: there is no Fawn, no Sam Collins (though Jerry Westerby seems to have inexplicably taken on some bits of his role), no Max ("I drive for you, shoot for you, what the hell?"). It must be hard compressing a complex novel like this into two hours of screen time, so maybe some loss is inevitable, but you can't help mourning your favourites.
And it's another film which doesn't know how to end: I was looking forward to the moment of unmasking, which is not actually shown, and to Colin Firth doing his big final number about why he did it, which is skated over very rapidly. And the end follows almost immediately. I think one of the last bits is meant to show Ann coming back to George, but I'm not sure.
Basically still worthy of your time, I think.