I've used one for few months and they work fine. the biggest draw back is that most Amiga platformers use the UP direction on the joystick to jump, not the fire button. This is very annoying. I've read that its possible to solder a wire from the up pad to one of the buttons so that pushing that button acts like JUMP UP, but I haven't done this. I've never come a cross an Amiga game that needs two buttons though, so can't comment there.
And yes a joystick makes most games unplayable for me as well, but apparently there are many others who prefer it this way. In fact, I play a lot of retro games under emulation eg genesis, SNES, NES Amiga using a PC analogue stick and I find it so much easier and have got my highest scores ever that way.