a front and rear u-joint, with no center bearing. the ACPT driveshaft has had no vibration problems whatsoever during the 7 years that i've had it. the only issue that came up was that the stock hex bolts weren't fully threaded and weren't long enough to connect the rear to the diffy. ACPT remedied that by sending out new bolts as well.
I've been quite happy with my ACPT shaft. I noticed a small difference... kind of like with a lightened flywheel... only there were no driveability issues (since the lightened part is after the flywheel).