Oh Yeah... IMHO, a big difference. All those bumps jarring the spine at speed, and I would have a sore back after every ride. With the seatpost, no sore back. (and, the backside thanks me too!) You'll still want to put your weight on the peddles when you know you're going over bumps or potholes, but, all the 'hidden' bumps that added up (at 25-30 MPH) are pretty much gone.
(Then I swapped bikes, went with a Trek Navigator with front suspension too, and it's a world of difference better.)
Note that there's a bit of adjustment. You'll want to put the seat an inch or so higher than you would with a fixed height, because it'll settle when you put weight on it. And, it'll take you a bit to get used to the seat 'squirming' a little under you, as it can feel like the seat is a little 'loose' on the seatpost, even though it isn't. The more expensive seatposts, like the thudbuster, apparently don't have the 'loose feel' that the inexpensive units do.