Blue foam pads are for young straplings with strong bones who don't need much cushioning for sleeping. I highly recommend them because they are:

1. cheap as heck
2. light as heck
3. indestructible
4. great for insulation against the cold ground
5. relatively compact

So they are not terribly comfy, but five out of six ain't bad at all.

Folks can roll up a blue foam pad and strap the bulk onto the outside -- or fold flat and slip it right into your backpack: fold the pad in half lengthwise, then in thirds to match the size of the backpack. Weigh the pad down with books (dictionaries, encyclopedias) for 1-2 days. The pad will "remember" this position, making future foldings a snap. Folded flat, just slip the pad into your backpack -- works great as a virtual frame for frameless packs -- or additional cushioning for internals. Either way, once folded flat, it is very space efficient.

And yes, the REI pads are softer than the Wal Mart ones.