currently most popular physics engines like tokamak, novodex, meqon,... seem to use linear time (O(n)) iterative solvers. compared to those newton is a bit slower but more robust (which is the more important thing with a physics engine in my opinion because it avoids lots of frustration)... ode currently uses an O(n*n*n) solver. i am sorry to say that but this solver isn't only much slower (do some simple stacking experiments) but this slowness also doesn't seem to have any stability advantages compared to the other engines (but there are other things like suboptimal collision detection which could make physics unstable)... as always i might talk nonsense!