I am currently using the external coax antenna with my Omnia at work. Without it I get a dead battery from constant searching for signal. With the antenna, I get 1 to 2 bars, which is enough.
The antenna port is in the dumbest possible place they could have put it. But then, if they really wanted us to use it, it wouldn't be under the back cover, hidden by tape. I drilled a hole in my back cover to make it accessable, saved the rubber plug from my xv6800 to plug the hole when not in use. However, when I plug in the antenna, I can't put the phone in the desktop cradle because the antenna is in the way. I'm hoping when we relocate our office, there will be better reception and I won't need it. I bought an extra back just in case