Just replaced mine; it was really simple, the only part that could have been a little easier was putting the skid plate back on since I only have one floor jack which was holding up the transfercase/transmission. If I either had a second jack or someone else at home to help hold one side up it would have been a piece of cake. Even so, it probably only took me an hour (which included cutting off a nut the PO cross threaded onto one of the transmission mount studs).
As for the poly vs OEM mounts, can't really say if either is better. I've heard other complain that the poly ones make things vibrate too much, but I've never used one to experience it myself. Mine was making a bad noise on acceleration so I was anxious to get it fixed and ended up buying one from advance auto instead of waiting on one to ship. Seemed identical to the one already on my Jeep, just not in pieces. Its been working great so far; less vibrations than before and no more clunking when taking off.