Personally, I use Kontakt Player because it's the softsampler which plays the libraries I use. The reason I'd upgrade to full isn't for editing possibilities or such - it's only to get the additional content and be able to load more libraries. Otherwise, it's much too big and complex for my needs. Same thing could probably said of all the major ones out there - from Halion to Mach 5.
If I really have to create and tweak instruments, nothing beats Logic's own EXS-24 - as simple and easy to use as it gets, plus, it's perfectly integrated. I always wished Cakewalk would integrate a similar sampler. As an alternative, I used to rely on Disco DSP's High Life which was free back then.