Pros: Overall Quality, Ease of Opening, Blade Material, Blade Sharpness, Lock Ease of Use, Pocket Clip, Lock Type
Hasn't left my side in 5+ years
This has been my EDC for the last 5+ years (maybe 6,I can't quite remember). It's EXTREMELY solid, and the m390 has become my favorite blade steel (I own a couple other knives with this steel). The first time I really put the edge truth it's paces was a few weeks after buying it.
I was helping a friend lay hardwood flooring. We realized that we had no replaced blades for the utility knife and it was a holiday so no stores were opened. If you've never done flooring, you need to lay rosin paper down before the flooring. Think construction paper, but several times thicker. Almost cardboard stock.
Typically you have to replace utility knife blades after cutting 10-12 pieces. By the end of the day I saw my friend out of the corner of my eye run the edge over his forearm. He yelled to me across the room "Dude! What the hell is this blade made out of!?! It's still shaving my arm."
The only drawback to the steel is that being able to hold an edge an almost supernatural amount of time is that if you let the blade go dull, it is going to take some work to put a new edge on it. This isn't an issue for me because I always keep my knives nightmarishly sharp. I maintain my edges, so I don't really ever have to put a new edge on it. I've done a true "sharpening" job only once since owning it (as opposed to honing it, which most mistakingly think is sharpening). I have diamond stones and paper, and though it took more time than my tool steel knives, it wasn't that bad (though if you only have ceramic or natural stones, you may need to clear your calendar, but if you can afford $200 for a folder you can afford some good stones).
Only real complaints is the lack of jumping to allow my thumb to get a good purchase on the spine of the blade. I use the sliding lock to act as jimping, but it's small and I can only use the back of the first knuckle instead of the meat of my thumb. Also you need to be careful when adjusting the tension of re folding mechanism. If you loosen it too much, the spring will come out of place. If that happens not only will the spring assist not work, it will start to open in its own or won't close correctly. This happened when I first got it. I sent it back and they fixed it. The bad thing was that I didn't tell them not to sharpen it, which they automatically do if you send it in. This thinned the blade more than I wanted, especially for it only being a couple months old. The upside on that was a couple years ago I was cutting a piece of drywall tape, and a live wire that wasn't part of the normal electrical system popped and took a large bite out of the edge. They replaced the whole blade, with anther m390 for $25.
Also, although the aluminum bolsters look really cool and reinforce it at the hinge, it does add weight. To be honest, if they made one that has the normal G10 w/o the aluminum bolsters like the 580 with the m390 I'd buy it. Im thinking of buying new scales for a 580 and replacing it if they line up.
I've bought and made dozens of folders (I'm a blade smith on the side), but this is still the one I go back to.