While one review here says that nostalgia is causing people to overrate this TV movie, I'm inclined to think that it's making people underrating it, because they're forgetting that, as funny and brilliant as the show could be, and as great as Tony Shalhoub was, the series always flawed and towards the end could be genuinely bad.
In the movie, Monk, an brilliant ex-detective with diagnostically fuzzy OCD is at the end of his always short rope. But when murder happens close to home (I really hate "this time it's personal" plot lines, but what can you do?) Monk comes out of retirement for what he's determined to be one last time.
As always, Monk unsteadily treads the line between character-driven comedy and cartoonish laughs. Monk sometimes responds as someone trapped in a world of obsessions, and sometimes just acts like a weird two-year-old having a freak-out.
In short, the movie, like the series is sometimes smart, and sometimes irretrievably dumb.
The mystery itself is decent though unexception - I figured it out way before Monk. The returning characters are decent although underutilized (except Randy, who is overutilized). The new characters are bland and add nothing, which is unfortunate.
Shalhoub, though, is still great, and his dry, anguished weirdness is still funny, even if it is the same shtick he's always done.
If you're expecting more than an overlong episode of the series then you'll be disappointed, but if you just want to see one more episode of Monk, this is worthwhile.