I wasn't going to buy a change table when I was pregnant with DS but it was mislabeled at the store for $11 instead of $110. So I picked one up. I have used it exactly once as a change table. But it does make a great way to organize multiple packs and sizes of diapers....
As for the car seat thingy, I would never use one that goes under the baby. They're dangerous. You should never put anything under baby that doesn't come with the seat - that includes 'after market' padding for the shoulder straps too. Car seats are crash tested the way they're sold to you. (See http://www.tc.gc.ca/eng/roadsafety/safedrivers-childsafety-notices-d200401-menu-336.htm
for more info and to see that I'm not just making stuff up :-)) I do have one of those bags in my cheap stroller though - the 5pt harness fits great though the velcro on the back and it keeps my LOs cozy in the winter when we're out for our walks.
Nothing wrong with the covers that go over the outside of the seat though. They don't mess with the integrity of the seat.