The only CRKT I've kept thus far is the Viele Wasp. Ergonomics could have used some improvement (more rounded/bevelled edges), but quality is great. The AUS-118 steel is amazing...I wish I could find more knives made with this (there was a limited run of M16s made with AUS-118).
The My Tighe and Mini My Tighe are also very nicely made. Solid lock-up, no blade play, and super fast AO. Nice attention to detail and F&F. My only gripe with it is the 1.4116 blade steel. If it were AUS-8, I'd have one, or two, already.
I'm looking forward to the M4. Uses AUS-8 steel, which is one of my favorite steels. But it's a liner lock, which brings me up to the main reason I don't like most CRKTs, including the M-series. CRKT's liners tend to be fairly soft and will travel fully across the tang in a matter of a dozen or so aggressive flips of the blade (not good considering the M-series are made to be flipped open). I've seen brand new M-series (M16 and M21) out of the box with the liner already at mid-point in the tang with a soft flip. A hard flip allows it to travel fully across. Any further wear on the liner and vertical blade play can start, or in the case of a thin liner lock like the zytel series, the liner could get stuck in the gap where the spacer is. I love the M21 design and would have one (like the AUS-8 steel) if the liner lock were harder and more wear resistant. Once the M4s come out, I'm going to play with a few to see how the liners are. I really like the M4 design, but won't get one if the liner wears quickly like the M-series. Considering the price of the good M-series and the M4 (street price around $40-$60), there's a lot of Kershaw and Benchmade liner locks in the same price range that take a lot longer for the liner to wear.