When done right, I tend to fall into the latter category. Unfortunately, this is not always the case. Some artists slide more easily into the box than others. Much the same can be said for the original 4-disc Led Zeppelin box. No Zeppelin fan could ever be satisfied with what was and what should never be omitted.
Some boxes simply go too far.
Besides, these sets tend to ditch the original artwork and sequencing, which just pisses everyone off. Track selection is really the most important and often the most labored point. Its not enough to string a bunch of singles together. Everyone has their favorite song and a lot of you are bound to be disappointed to see that the compilers don't share your tastes. Thus, it's sometimes better to stick with the individual albums.