The Unit portray a small team of the best of the US Army's Delta Force led by the super cool Denis Haysbert. Plenty of action and James Bond style gadgets/weapons. The stunts, shoot outs and battle scenes are excellent. Haysbert's dialogue is always razor sharp. Sometimes it shows how operatives qualified or the Sere training they go through (vicious). Most episodes are set where you expect: e.g. Middle East, Eastern Europe, South East Asia, Latin America, Africa.
Each season is very cool in its own way. Some highlight episodes for me personally were: First Responders, Play 16, Dark of the Night (total war episode), Two Coins, Silver Star... and I'm not sure of the titles, but episodes 7 and 8 in season 4 were incredible as well.
I thought the new characters in Season 4 were good but not great. Red Cap is cool but Whiplash is not so cool (there's only one Hammerhead!).
One criticism: The producers should have balanced less focus on the soldiers wives at home and given more attention to the missions. I found the scenes with the wives got in the way of the missions as you were watching them unfold. Many of these "wife" scenes don't build tension they just cause frustration: Example, Change of Station episode - Season 2 - amazing action and mission but the wife scenes had me diving for the dvd fast forward button. The theory that family drama works well with action/gangster style programs might be true (e.g. Sopranos) but it doesn't necessarily translate well into military style programs.
The mission and the men is why we really watch the Unit. Keep it simple with lots of guns, action and bad guys for Snake Doctor to split in half and people will keep watching it.
Here's to Season 5... if it ever gets made :)