If you can simply accept the sheer implausibility there's more than a little fun here, with Gerard Butler as a near-supervillain holding an entire city hostage from his prison cell. Some really nasty and imaginative killings and Butler's maniacal performance keep it ticking over against all probability.
A very-well made action thriller with a heart as Luc Besson for once gets it all just about right. It helps that Jean Reno and Natalie Portman have genuine chemistry in their double mentor/pupil relationship, and that Leon himself is a fascinating character (even if he's about as Italian as Joe Dolce). As for Gary Oldman as Stansfield he's so completely over-the-top insane that he somehow works perfectly, the old Andy Robinson gambit coming off perfectly.
That Besson later screwed the film over with a flabby 'long version' summed his career up nicely.
A remarkable film - Children of Men was always going to be a tough act to follow but Gravity manages to be even better. Beautiful and terrifying at exactly the same time (the debris strike on Explorer is balletic and chaotic, more shocking for not having inaccurate tearing metal noises), made with incredible technical skill and held together by two astounding performances from Clooney and Bullock.
Cuaron also knows that scientific accuracy is only worth a damn when it's needed by drama and that accuate-ish is accurate enough, and also that you can make a space epic without needing to make the thing three hours long.