Just a bit of my recent experience with Soundcloud:
First, ProTools 10 includes a "share to SoundCloud" option when exporting your mixes.
This I find very timesaving and useful.
Second, I have found that leaving headroom wothin your mix helps the upload encoding process IMMENSELY.
Usually, I leave anywhere from 3 to 6 dB of headroom of the peaks to the maximum volume of the file, 0dBFS.
Third, it really is helping me to realize where the mixes stand after being encoded into SoundCloud. Things that are phasey really crap out, things that were too "subtle" in the mix are completely lost, so generally I can decide to adjust my mixes a little bit according to what is coming out of the SoundCloud encding process.
I recommend people here start using this service, or anything like it, as more and more music is being accessed online. It is the delivery media for the 21st Century.