I found machetes good for trail clearing and cleaning out sight lines for surveying.
Light brush and cane from thumb thickness up to about wrist thick, maybe up to 8 inches for poplar or willow.
Not for very hard woods or dead wood.
I found the style that they call Latin about the best.
The ones by Ontario Knife are pretty good examples.

I find heavier blades like the Collins Bolo style machete don't bite the same and tend to push the brush out of the way instead of clipping it.

What I don't like about machete is a lot of people will swing them towards themselves. You must be careful not to have a leg or anything in line with the swing, always swing away from yourself. One miss and you could amputate a leg or a hand.

edit: I find the 12" ones too short and the 24" ones just a bit too long for comfort. 18" seems about right for me to use.

I will also say that a machete is not really a general purpose tool for me. I use them almost exclusively for brush clearing on trails or for survey lines.
On trails it is just to remove the small branches that snag you on the way by.