I want to be careful what I post here.
1. Fred wants playtesters to discuss DFRPG and "get the word out".
2. Fred does not want too much information leaked and I'm under a NDA agreement not to spill any real secrets.
Having said those things, here are a few thoughts I have at the moment based on my playtest experince so far. I believe that nothing I reveal will be a secret, and will intentionally remain somewhat vague:
1. Go buy Spirit of the Century. Right now. If you don't buy it, at least download the SRD for SotC. I felt that I had a decent understanding of SotC until I really played it, and now I realize that it really makes more sense in action. The DFRPG will be very similar to SotC, so what are you waiting for?
2. The DFRPG is similar to, but not exactly the same as SotC. Start with SotC right away, however, to make the "learning curve" better later on. Some of the skills/stunts/aspects have changed a bit, but the vocabulary of FATE can be heavy if you don't spend some time absorbing it. That's why you want to start right now. That way, when the DFRPG comes out you'll be ready and the transition will be small.
3. FATE is really the way to play in the Dresdenverse. I know that I've been trying other game systems to simulate DF for a while now, but I can really feel things starting to click here. The FATE system is detailed enough to do what you want, but simple enough to allow you to play without heavy reliance on the rulebook. When I got my players to use skills/stunts/aspects, I would just ask "what do you think it does?" rather than consult the rulebook all the time and the whole thing runs so smoothly. I know in general how the system works, but the players can pick up what they need to know in a hurry and without huge prep on my part.
4. Fred and Rob do an excellent job of constructing a doccument. I know I've said this in other threads, but when I read the DF playtest docs or read SotC I keep saying to myself "Wow. I knew this, but it becomes so much more clear this way!" Every time I read SotC again I find some new nugget of wisdom that I missed the time before.
5. Maybe it's just that my group has done Swords-and-sorcery fantasy so much lately, but my players keep bugging me about running more Dresden sessions. And some of them haven't read the books or watched the TV show!
6. The playtest docs are clearly a ways away from being a final product, but everything I have seen so far seems to point to a really fantastic RPG when this is finished.