I got mine as a combined deal on CL for $55 (came with the largest Topeak bag, the one with zip down panniers in the side pockets) and bought the add on pannier frames for support.

As mine was used...It didn't come with the inserts for the clamp so I had to make do with what I had on hand.

I had lateral movement problems both with inner tubes and ductape (wrapped around seatpost).
Then I had the bright idea to place several sections of inner tube parallel to the seatpost and it remained rock steady afterward.

The only thing I didn't like about the seatpost mount is that it placed the bag pretty high behind the seat making things a bit top heavy (that and I have a big butt and felt that I was sitting half way on the bag)

I have Topeak Explorer racks on my other 2 bikes and love the "quick change" mounting system.
It really makes moving the bag back and forth between bikes a snap.

Edit--Oh, yeah...the tail light thing wasn't an issue after I got a Superflash and clipped it to the loop on the bag (nice and secure). One tail light for three bikes! Just move the bag.