Fellow is very fast, but is only usefull for old games.
Just thought I'd add a bit to that. Fellow is fast when doing the chipset emulation. It is therefore well suited to old games, and will run some stuff that UAE will not.
However, last time I checked Fellow has no JIT, so CPU emulation is actually quite slow, meaning it is not well suited for applications at all.
For old games, Fellow makes a good solution. For anything else, forget it.