advantages - more (especially off-power) steering, no diff to unload during hard cornering, front end pulls adding steering out of the corner
disadvantages - more (especially off-power) steering, more mass/weight, makes using brakes very difficult (car wants to trade ends in lower traction conditions), easier to bend/break axles as there is no diff in the drivetrain to transfer force to
in a nutshell, a one way adds steering. you'll notice the effects of a one-way the most when entering a corner. under hard cornering, a diff can actually unload the inside tire resulting in 'push'. with a one way, the car tends to turn in much more aggressively. like anything else, it's a tuning option. unless you're racing at just one place, and the one-way is not the hot setup, you should have one in your pit box.
as for who makes a good one, the version 2 tc3 one ways from associated are very reliable. they are manufactured by yokomo, and were originally used in the ntc3, then marketed as a 'heavy duty' option for the tc3 (uses different outdrive cups than ntc3). i believe currently they're just labelled as a plain old 'one way' (no more 'heavy duty').
price, between 52-60 depending on the shop.
if any of this is incorrect on the making/manufacturing portion, please correct me, it is all second hand info.