It's an interesting film; good in many respects, but not so good in others. There's a fair amount of material removed, and after reading the book this does make the film feel a bit dumbed down. In one or two places there are small changes, which do generally weaken it. I'm fairly sure Le Carré never refers to HIV in the book, only to TB, and whilst I can understand the desire to point out the link in an African context, it explains a little the rather confused review of the film on Newsnight a few weeks ago. For me, part of the book's effectiveness is that the pharmaceutical storyline focuses only on TB, a disease currently relatively controlled or eliminated in the West, whereas the introduction into the storyline, however briefly, of HIV, dilutes the global disparity somewhat. A more valid observation from the Newsnight review was the comment that the film (and the book) are very black-and-white, unusual for Le Carré who normally writes in shades of grey. It's interesting that there is scope to explore relativism here, but it is barely done. After all, it's not as if global pharmaceutical companies set out to kill people with their drugs, it's that they are unwilling to change their actions if such a side-effect will hurt their bottom line.
The photography is mixed. Lake Turcana is spectacularly alien, and the scenes in Kibera are effective. On the minus side, a lot of the shots are done in the Blair-Witch-style of running around as if with a hidden camera in a documentary. This was never George Smiley's modus operandi and it isn't Justin Quayle's either.
Overall, if you liked the film you should read the book; if you liked the book, the choice is yours as to whether you see the film.