I have made some research and found that bullet, ode, newton and some others are open source physics engines that should be portable enough (but I have never tried to comile/use anyone of them on phones). I am writing my games for mobile platforms in C++, so the engine should be C or C++. I need a fast engine, since mobile platforms have limited resources. I need a free engine. A good design would be nice to have too.
What engine is best suited for my task? What I really would like to hear from you is your direct experience.
Documentation and support (for example, forum or an IRC channel) is a really important aspect to take into consideration.