Well, I bought a seatpost rack. Interesting results
 

I bought a Topeak seatpost rack. Very light and looks cool. Today, instead of using a backpack I used a leather briefcase/bag and attached it to the rack. It definitely made a positive and negative difference.

1. It made me a LOT more comfortable. After just one day of commuting I already think, "how did I ever handle that horrible backpack?"

2. The bike doesn't climb as well or handle the same, but a lot of that is the way the bag is sitting on the rack. The bag is hanging out the sides quite a bit. I need to find a way to narrow the cargo to stay more in line with the bike (and I need to lessen the weight by reducing what I take back and forth to work--being a teacher makes that a challenge).

Anyway, I'm glad I bought the rack, but I need to work on it.

Hope it doesn't flop on you. Good luck.

I use the same type of setup for my lunch but I also wear a backpack. I agree on both accounts - it is far more comfortable however more weight over the back tire (like when I had a milk crate attached) negatively affects bicycle handling. It would probably be improved by figuring out how to keep the weight lower and more evenly distributed side to side. I suppose you could always buy panniers and a regular rack.

What does that mean, flop on me?

My rack could lose its erection, so to speak?

I just need to lighten my load.

Seatpost racks are prone to swinging off to one side or the other when they're over loaded. This is dangerous, since the rider cannot easily compensate for the sudden balance change. If the load is small, the risk of this happening is small. Most seatpost racks claim a weight limit of 20lbs. I've known a couple riders who've had trouble with seatpost racks swinging around right at the limit. For safety, I'd be reluctant to load a seatpost rack much past 10lbs. Since it's easy to misjudge the weight you're carrying, I am not keen on anyone using a seatpost rack.

A standard rigid rack can't swing like that, so it's a safer design.

If I were riding a bike that couldn't take a standard rack, I'd probably go with an old fashioned saddlebag and a bag support. Harder for stuff to swing around. Pricey, but not as pricey as fixing me if I'm in an accident.