I've been meaning to play around with BulletSOP, a new implementation of Bullet inside of Houdini. Yesterday I did my first test with it and came away very impressed. It's a very simple test, I copied a cube 25,000 times and ran the sim for 500 frames.
The simulation took only a half an hour for all 500 frames, I was blown away at how fast this was. Just getting 25,000 cubes into DOPs can take a long time, let alone simulate them!
I rendered it out using Mantra's Physically Based Rendering, and the times were only about 1.5 to 2 minutes a frame, but when you have 500 frames and 1 computer it still takes forever to finish the thing (about 14 hours at 1280 x 720).
All and all, I'm super impressed and I'm excited to try simulations on more complicated geometry.