I suppose Budapest is probably more cinematic than Brno, in which Jim Prideaux's mission goes badly wrong; likewise Istanbul provides a more dramatic backdrop than a Greek island backwater (though a quick check reveals that both adaptations differ from the book, which relates this part of the story in Hong Kong). Some of the flashbacks are re-arranged, which doesn't always add coherence to the story. It's unimaginable that Control would take his work home to his flat - his wife always thought he worked for the Coal Board. The language is stronger; Peter Guillam is gay; the ending lacks any ambiguity whatever. But some things, such as the story about the cigarette lighter, are virtually identical.
Overall, the film is a half to two-thirds shorter than the BBC adaptation, so I suppose a lot of things have to be either cut or compressed. Yet the film doesn't obviously show either of these attributes - there's certainly plenty of space in it. It chooses to revel in drab 1970s colours, rather than in John Hinde technicolour. It looks odd for the Circus to be laid out in such an open plan - even for a typing pool - though the futuristic anechoic meeting room pods are perhaps a more logical depiction than standard offices.
The cast has been well chosen, particularly given that the dialog is generally quite minimal. Among the supporting characers, I would particularly highlight John Hurt (Control), Mark Strong (Jim Prideaux), Kathy Burke (Connie Sachs) and Roger Lloyd Pack (Mendel). I'm not so convinced by Toby Jones (Percy Alleline), though it's a difficult role. David Dencik (Toby Esterhase) is not quite as successfully weasly as Bernard Hepton was; Ciarán Hinds (Roy Bland) has so little to do as his evidently eponymous character. Gary Oldman does a fine job as Smiley - sufficiently distinct from Alec Guinness, though with a voice in his opening line (when it finally comes) that could easily be mistaken for Guinness; and my own opinion is that Colin Firth does a better job as Haydon than Ian Richardson did. If I had to choose only one adaptation, it would be the BBC's, mainly because Guinness is so definitive, though the world is enriched for having both.