The movie is worth to watch, even better to watch it after watching the series.
In the series, everything happens to fast, that many things, e.g. the chemistry, the reasons behind of why the Agent let In Soo does what he wants, etc. were lacked. However, in the movie, everything was there. Those added scenes with more details make the movie more meaningful.
I think Korean makes very good BLs (Mr. Heat, Where your eyes lingers, Wish You): great story, good storytelling, excellent acting, not-too-much-drama, good cinematography, ...
Very much recommended you to watch it.