From what I've heard they generally sound pretty good. However, they're not up to gigging, simply because of how they're built. Plastic cases, the knobs turn trimpots mounted directly on the PCB and so on.

And please, please, don't buy a cheap multiFX unit. The cheap ones are the reason why so many guitarists hate multiFX. Definitely don't buy one that has a plastic casing - it's not designed for serious use. A secondhand GT-3 would be a much better bet for about the same money.