It is best that you do a search on serrations (click the search button, type in serrations, and read for a few hours!)
But, here's the skinny:
Serrations make for aggressive slicing cuts, showing their advantage when cutting fibrous material (rope, lots of cardboard, plants, etc.) Serrations will hold their edge longer than a plain edge. Serrations are easy to sahrpen with the right tools. The right tools are things like the Spyderco Sharpmaker 204.
Some say on fighting folders, a combo edge is great as you egt soem ripping action in. Soem say combo edges are great for utility, as you get a plain edge section and a serrated edge section.
My own recommendation/preference is for a coarsely sharpened plain edge. Sometimes I like the serrations on my combo blade, but MOST of the time, I'm using the plain edge. I'd rather have 4" of plain edge instead of the 2" of plain edge I use all the tiem now and the 2" of serratiosn that see occasional use. If you cut fibrous material lots, get yourself a fully serrated blade and carry a plain edge. Many "old timers" here will express a preference for plain edges.
Make sure you read the sharpening FAQ here. You also nead to realize the difference between a polished edge which is best for push cuts and a coarser edge which is best for slicing.
If I were wanting a field tool, i;d pick the Zambezi and reprofile the edge. If I were wanting a concealable fixed blade, I'd pich the yar and probably redo the edge as well. Don't let a factory edge sway you from buying a knife.