I think if you're going to have first person dialogue outside of orders(eg. "follow me," "attack," etc.) then it should be carefully tested. Although it all depends on what sort of first person voice acting you're doing. If dialogue affects the actual story and can change how the game is gone through, then it should be tested to make sure that every sequence lines up and whatnot.
First person voice acting, overall, is a great thing. It really is a shame that a lot of games don't take advantage of it. As Zaphod said, Vietcong uses great first person voice acting, but it does not go beyond the usual "cover me" and "attack" dialogues. Something more profound, like, "take your team east 200 meters and come north" instead of "go 100 meters west and head south" will undoubtingly affect not only the gameplay, but the story as well (if the team is needed in the story, and they get killed by following those orders, for example).
Anyway, yeah. If a game can make some really good use of first person voice acting other than simple orders, I say go for it. It adds that much more depth and realism, and for a tactical game (such as your military game you mentioned), it'd be a VERY helpful feature to have and could assist the player in more ways than one.