I think that's probably why I like these crossovers so much. The outside pov on the Winchesters, how the LEO don't see them as the heroes we see them as, but as the criminals that the cops on SPN tend to see them.

The fun thing for me is to have that idea of how regular people/cops see what the Winchesters are doing, and possibly the shock to the system they get when they realize they're wrong.
SPN for all that it's rooted in horror and monsters, also has a deep seat in reality as well, with characters like Victor Henriksen and all.

Here are some of the NCIS/SPN crossovers I've got bookmarked

Mixed Priorities
NCIS Team meet Supernatural over a case involving a house fire and a missing Staff Sergeant

This one's title unfortunately isn't quite correct, since I really wanted to see more of it, but the fic itself is awesome
Five Times Dean and Tony Bumped Into Each Other
Tony has to pretend to be a fellow criminal to get close to Dean

Agents and Hunters
People who work in Abby's lab are never what they seem. And Sam is no exception.

Volatile Compounds
The Winchester boys are used to just cranking out fake IDs and getting access to anything they need when they're hunting something. They've rarely come across anybody in law enforcement who's truly competent. It really, really sucks when they do.
(there's a sequel to this one: Retrieval

Always be specific when you lie
Castiel helps Dean out when he's in NCIS custody

Re the five times fic -- the five bumps happen all in the same fic, mostly as they're sliding on the bus or stumbling around in the woods or yanking on linking handcuffs I was a little disappointed to realize that myself, but really, how could they follow that one up?