I've had one for 2 1/2 years and it's great! I run 3.23s in the back so highway cruising is very nice.
Installation was a breeze. On my 71 b-body all we had to do was hammer the trans tunnel a little bit to make some space, but barely. No cutting.
I love telling people I have a 6 speed auto.
A number of people have said that the gear splitting is a waste of time, I have to disagree with that. I run mine in manual mode and the kick I get when going from 1st to 1st/OD is incredible. When first winds out and you're near 6000rpms, kicking in the OD takes me down to a nice spot in the power band, and the car revs quickly through another 20mph or so.
I will agree that shifting from an OD gear to a higher non-OD gear is not clean. You have to first shift to the higher gear, and then once it engages, take off the OD. But in my experience it only takes about 1/3 of a second for that, so it's no big deal.
Jim