Whatever is necessary to accomplish the task.
As everyday hunting attire? No.
As a tool for specific situations? Yes.

I wear them all the time.

To the mall, to the post office, to work, as pajamas. They're also good for hunting, but in my opinion unnecessary. ALJoe is right about the blue jeans. There is, however, something to be said for complete concealment...it really can help.

My grandfather killed dozens of deer and turkeys wearing nothing by brown duck and red & black plaid Woolrich.

I wear camo. (I like Predatory brand but I suspect mostly because it's different looking than the others.)

But, I always remember his advice. Don't silhoutte yourself - sit with your back to a tree, bank, etc. and keep a little brush in front of you to break up your outline. Most importantly, don't move.

If looking like sasquatch gives you confidence as a hunter, have at it. It can't hurt as long as you don't get the ghillie strings wrapped in your gun action or bow cams.

Hi...

A good suit, worn by a person very familiar with it, and under the right conditions, can keep himself out of sight of other humans.

Otherwise, staying out of sight of your quarry and staying STILL (and downwind) would be of similar value when game hunting.

Incidently, green (and yellow) are the two colors that deer CAN see.

Camo really puts dollars in the dealers' pockets, and I deem it quite unnecessary under most circumstances.

During hunting seasons...especially during rifle hunting, you're less likely to be shot by some over eager hunter if you can be SEEN...wear orange...!!

I think it depends on your situation. I use the Cabelas ghil-leaf and TCS stuff when I turkey hunt and archery deer which I do from the ground. I know it helps as I have had multiple close range encounters this year where the deer knew something was there but did not bolt, just moved off.
For rifle I don't think you need one and a full ghillie is probably more hassle than it is worth if you have to move around.

I find them too bulky and they catch every brier and stick in the woods.

Predator Camo not Predatory...that's the term for the marketing campaigns mounted by the companies making camo. (Another term for their marketing is "a sucker born every minute"...and I'll admit I'm one of the suckers.)

In bow season and a ground blind. Nothing better. In a tree stand during bow season when a few leaves are still on the trees, Yep. And thats just about it.

SquirrelGirl: Thats just plain wrong. :-)

If you have ever tried one or a version of one you don't need you ask, they work really well.