Just finished re-watching all five seasons. It is a fantastic show. The character arcs, the minor stories, all played out brilliantly.
Season 5 did sag a little but maybe that's because a) it was shorter than the others, b) it was clearly the final season and they were tying up stories rather than playing the long game, and c) the bonkers McNulty storyline. McNulty was hardly in season 4 and now was back and up to ridiculous antics.
I watched it first when it was originally on, but this is the first time I have gone back for a re-watch and enjoyed it much more this time around.