We're now hopefully in the final straight of Fedora 21 testing! We're actually relatively on top of the schedule this time around, as compared to previous releases - by Thursday we'll have had nearly 48 hours to test the current (and hopefully final!) package set (across the RC2 and RC4 composes).
If you want to chip in and help out with validation testing, all the info you need is in the RC4 testing availability announcement. Please do be aware that TC and RC Fedora builds are strictly release validation testing builds, these aren't public pre-releases, so don't ask why they're not on the mirrors or why the announcements aren't nice and PR-friendly, this is part of the Fedora testing process :) If you just want to use Fedora 21, sit tight and wait - right now signs look good that it'll be released on Tuesday, as per the planned schedule.
There's a couple of bugs I wish we'd had time to fix and a few rough edges left on the Product stuff which in an ideal world we'd have had time to smooth out, but I think 21 is a really nice release in general and a pretty solid first attempt at the Product work overall. We're looking forward to getting it out there!