I have one
I grabbed one of those adapters, unfortunately, I cannot remember where. One end (FME) fits into the back of the phone with the 90 degree plug, the other end on mine is a TNC connector. I got that as I had a couple of spare antennas that use TNC.
It does improve the reception. I use the antenna's in the vehicle when on the highway to maintain reception. (I have no problems in the city)
Depending on your device, you may want to update the Radio ROM, if you haven't already. This doesn't flash/reset your phone, but does update the radio. This might resolve your problem without spending any money.
I am using the 2.47.11 Radio ROM and it did make a difference. My device is the T-Mobile MDA (Wizard). In a particular location, where I would get zero reception, I now get one bar...and that's all the difference I need.
P.S. If you do get this adapter, be careful when plugging it in and during use. Remember that FME connector is attached to the circuit board.