Having recently moved from Logic to Tracktion I needed a decent suite of ordinary fx since those supplied with Tracktion suck. Just things like delays, chorus, flange, compressor, a reasonable reverb and that sort of thing. Quantum-fx supplies several versions of all the basic fx I need, as well as some excellent creative oddities like 'ambient pads' and 'loadza filters'. In fact, these more distinctive fx are where quantum really shines, and the ability to build your own fx in a stable modular environment is mind-boggling. Some of the fx provide tempo control, added in a recent patch I think, and tempo syncs are available in the workbench too.
As an added bonus, ab-audioware has a library of new fx built by the company and third parties. Of particular interest to me are the new mastering tools, which are on a par with the best mastering processors I've heard. Sound quality is faultless, reliability perfect and the CPU cost of even extremely complex fx is very economical.
Quantum FX does look a bit ugly, but once you get your head around the parameters for each effect, it becomes just a tool to do a job and the displays are clear enough. Like someone else said you can replace the graphics if you can be bothered, but its fine for me.
Quantum-fx isn't exactly cheap, but the quality and stability of all the fx seems to be well above that of freeware equivalents and as a one-stop set of usable fx, its unbeatable. Luv it mate :)