First I would like to let you know that I just saw on the Rift website that the 4th betaWarfronts is schedule to start on Friday, Jan. 7, 2011 at 10:00 AM PST (18:00 GMT) through 10:00 AM PST (18:00 GMT) on Monday, Jan. 10, 2011
To answer your question, I've was lucky enough to be invited to beta 2 and beta 3. I really enjoyed playing during both of those events.
Souls systemAt first when I saw the post about the first patch of Beta 3 saying that we would have all 3 basic souls when leaving the beginning area I was septic about this decision, because I was afraid that Rift would do like many MMO and give everything easy without any work. But after playing beta 3 helped me understand better what the souls system was really about was. I've played a Paragon, with Reaver and Riftblade souls and really enjoyed the many options that were offered to me.
The graphics of this game were the best I've seen in MMO's, I've played EQ, EQ2, FFXI, Vanguard and World of Warcarft and so far, I think RIFT has a very solid product even before the release date which as per EBGAMES pre-order would be schedule on March 1st, 2011. (Can't wait) However, Direct2Drive or Steam do not have pre-order or release date on their website yet.
The MEGA Rift invasions were really original and fun. The dev team probably has the best load test scenario during those. It was had on my system at time, my 5870 would give me a 35-40 fps average, but during those mega events or when playing with 30+ other ppl it would go as low as 7-10 fps which was making the game a bit harder to play.
That been said, I really hope to be part of beta 4 and all other beta, maybe Zam could give me a little VIP key to help me a bit ;0)