I've been carrying a knife daily for over 30 years and mostly daily for well before that. I mainly considered the knives I carried over the years as tools rather than weapons but always considered them weapons of last resort. I'm no knife expert and have had very little training with them and would not readily resort to using one as a weapon but there is no doubt I'd use it if there was no alternative. I have no doubt that if I was engaged in a knife fight with a "knife fighter" I'd lose. But if I'm fighting for my life and there is no alternative I'll darn sure do my best to use one. Telling someone to not carry a knife unless they've had extensive training is IMHO arrogance. I'm not discouraging training, I'll even recommend it, but lets be realistic.

That said, I always carry a knife with a blade of less than 4" and usually about 3". It's just more practical and makes a more useful tool and is still big enough to change somebody's mind.

You must be very cognizant of the laws regarding knives. They vary a bunch. I generally don't carry even an assisted opening knife when traveling out of state.

The cheap Gerber I carried for years was sort of a Spiderco copy and I could open it in a flash one handed. It had (has if I can find it) some thin synthetic plastic handle that had some flex but that thing has taken some serious abuse over the years including being used to hold up my old connie motorcycle's kick stand in soft dirt. Hard to sharpen but held an edge well. It's cut tons of rope, cord, boxes, tow straps, seatbelts and fingerprint tape and who knows what else. It's even carved a toy boat on a stakeout.

Anything much bigger than a pen knife should not be carried concealed in any way. Having a synthetic push knife in your pocket is a sure CCW charge. Use common sense.

There are bazillion cheap knives that are plenty good enough. Kershaw does seem to put out a good variety of adequate but inexpensive knives. I do have assisted opening and automatics I carry but with care and not out of my home territory. Many knives with thumb studs on the side or top of the blade can be made to open lightning fast with some effort. I prefer drop point non-serrated traditional blades because they're easier to sharpen. I prefer ones that are not hollow ground as I feel they are stronger and although harder to sharpen than hollow ground they hold a proper edge longer. I know I won't carry and automatic or and assisted opening knife that doesn't have a lock.