We have a Vetus Bullflex and have been very happy with it. With the right engine mounts, it substantially reduces engine vibration. The August 2004 Tech Notes has a write-up describing the installation. For boats with the M-25, in addition to the coupling, replacement of the stock mounts with Vetus K75's on the aft and and K50's forward seems just about optimum. For M-35's, K75's all around (although there is no harm in four K75s [which are slightly stiffer] with M-25 equipped boats either).
Keep in mind that the installation of a flexible coupling minimizes the concern with engine alignment, but does not eliminate it. The real purpose of the coupling is to reduce vibration transfered to the hull through the shaft log and strut, not to act as a "universal joint" for the running gear. The engine should still be aligned following normal procedures after the new coupling is installed. The Tech Notes article provides a method for changing the coupling with the boat in the water that requires the aft end of the existing shaft (the key way portion) to be cut off.