I cannot comment on the combat effectiveness of a tomahawk but I can say that its a much more useful camping, hiking, hunting tool than most give it credit for.
I have a growing collection of hawks that I really don't need, but I love the designs and what they bring to the table for the above mentioned tasks. I have a couple of Cold Steel hawks ( great for the money ), a ATC hawk ( amazingly light and effective ), a couple of custom's fromCoal Creek Forge ( awesome hawks and powerful cutters - on wood ), and a couple of RMJ's- a Shrike ( full size, not S13 ) and a Jenny Wren.
They make quick work of skinning and field dressing deer, are fun to throw when bored, cut small trees quickly and easily to make hunting blinds or make shift shelters, split the wood easily, can be used like a knife to whittle- again when bored while waiting for a deer to volunteer, and work pretty much like a large knife in a package that is often much lighter.
I'm not trying to debate the knife versus hawk thing, just stating that for me there are many uses for a hawk in the woods, one of my favorites is as a pickaroon so that I don't have to keep bending over to pick up firewood, a spiked hawk excels at this type of work- weird, maybe, but useful just the same.
Out of all the hawks I have I have to say tat my RMJ's are my favorites to use, partly because of the awesome sheath and the many ways that it allows you to carry the hawk ( shoulder strap rocks ! ) but mainly because of how extraordinarily tough mine are- I'd trust them with my life, and although that may not mean much to you it is as high of praise as I can give it.
My favorites to own are my Coal Creek Forge hawks. They're not only very tough and can chop like a machine, but they're just beautiful to look at and are works of art in my opinion.
As far as combat effective goes, let me ask any naysayers this. If things went sideways and you were out of ammo, the other guy had a knife or some type of bludgeon or melee weapon, and he was closing in on you, would you feel as concerned if you had a hawk? Anyone that can trim branches and chop would most likely be proficient enough to side step a swing and end an attack. Is that combat effective ?