I have a Spyderco Endura 4 Combo Edge, among others. This particular knife seems 'just right' for how I normally expect to use it, for tasks like cutting the occasional piece of rope or twine or vines from the trunks of trees on my property. The blade provides about 1-3/8" of serrations and 2" of plain edge out front, which is just enough of each to make this a very practical blade (for my purposes). I keep the plain edge portion as extremely sharp as possible, and it's very useful for opening packages/mail, cutting up boxes, etc. I think as long as there's enough total blade length (more than 3-1/2" or so, at least), a combo blade can be pretty useful. If the blade's any shorter, I would only want plain edge.
As my sharpening skills are continuously evolving and improving, I'm finding that I'm buying more plain-edged knives now. I've found that a plain-edged, high quality steel blade with a SCARY SHARP edge is just as useful (maybe more so) than a serrated or combo blade. But, having said that, I still have respect for a well designed combo blade. The Spyderco serrations are better than many others, in my opinion. Not quite as difficult or tedious to maintain, they're wide enough so as to make it easier to get a sharpener in there.