Besides "The Bourne Identity," "Ronin" is the only other recent spy movie I can think of that didn't feature skydiving, bungie jumping, skiing downhill while shooting innumerable bad guys, laser pens, cars with ejection seats, or silicone breasted women with names you'd be vaguely embarrassed to say in front of your mother. Most of the crap that passes for an espionage film has no plot or reason for existing other than to meet a quota of explosions and cleavage in order to draw the summer action film crowd.
While "Bourne" does not have a particularly deep plot, it is consistent and focused. The focus is entirely upon Bourne and how he is to deal with having no memory of his past, being hunted without knowing why. Some people have complained about being confused by the movie. I for one, do not need to have everything spelled out since in many cases this smacks of unreality in the first place; the essence of espionage is drawing conclusions from very sketchy information. If you can't handle a little of that, you probably should stick to Disney films or TV's Scooby Doo where everything is explained in the end. We never find out about what is in the case in "Ronin" and I can live without some information being filled in about Bourne's past.
To those who have moaned about the incompatibility of the book and the movie, seek help. There are probably several things that work in the book that would either be boring and take too much screen time to explain, or would be viewed as cliche to modern screen audiences. As I remember, I liked the book, and I definitely like this movie. I view any movie adaptation as an interpretation of the book rather than a translation from words to pictures anyway.
As for realism, most of the action scenes are believable, no super-gadgets are to be seen, no incredibly lovely models fall madly in bed with Bourne, and the hero shows definite signs of physical vulnerability despite a very high level of training and competance. As someone who has trained in martial arts for over 10 years, unrealistic fight scenes are a pet peeve. The fights in "Bourne" are fast, nasty, and very realistic while still being entertaining for the layman. (And yes, taking a gun away from some idiot who is standing well within your striking radius without getting shot is definitely doable, though I had serious doubts until we tested it for ourselves with plastic dart guns in the dojo several years ago).
While not being perfect, "The Bourne Identity" is, simply put, several grades above the typical spy film. Being focused on an individual level rather than involving itself huge political ramifications lends it another layer of respectability rather than detracting from it as some comments have implied since it remains a human problem on a comprehensible scale. The mysteries that are left are bigger mysteries for Bourne than us, and I think should be viewed as intentional omissions rather than loose ends. The implausibilities are kept to a minimum and the realism to as high a level as possible while still being spectacular enough to meet the expectations of the genre.