I got part way though a team game before the 1.52 patch (savegame didn't work after patch). I played as China/Qin with Japan/Tokugawa as my partner versus 6 civs on a standard map great plains on noble.
I ended up with a not too good starting position -- not in terms of what resources/terrain was nearby, but because I was sandwiched between four other civs. I shared a small border with my partner, but was essentially a border guard for him. This allowed Tokugawa a lot of room to expand, while I quickly ran out of space due to pressure from other civs.
For the most part I did enjoy the game -- the fact that my teammate had a strong military left me with the feeling that I could let mine lapse a little, allowing my smaller civ to focus on production of other things (going on the assumption that any attack agaist me would draw retaliation from Toku).
The problem was that throughout the game, I was and always knew that I was the weaker partner. My partner was ALWAYS the better producer, researcher, etc. I had no hope of catching up, and this left me feeling disappointed. The only thing that I had the advantage in was spreading my religion, which soon became the dominant religion across the world. But realistically, aside from a bit of extra $, this did not improve gameplay for me.
That being said, I admit that I probably played it wrong from the outset. I think that in team games you really need to accept the traits/advantages of your partner. I was so used to playing by myself that I think I focussed too much on working just MY civ.
Lesson learned: when playing as a team, make sure you always keep that in mind. It's a completely different game than playing by yourself, and if you don't work hand-in-hand with your partner, you'll being both down.