The cutting ability is very high, especially compared to modern, thick edged "tactical" knives.
I use an Opinel on a regular basis, both the stainless and carbon steel. Both hold an edge fairly well, not like my Sebenza or BM 710HSSR or 705 in M2, but very respectable.
The thin blade stock is the key to their high performance. For slicing cardboard a few swipes on a bastard mill file will put on a very aggressive slicing edge
A few minutes of stropping on a piece of saddle leather charged with lee valley green polishing compound gives it a very nice high polish edge, very well suited for push cuts, used mainly in food prep and wood work.
As to the issue of lock strength, I am probably not the best judge, as I often work with a slipjoint SAK, stockman or canoe (Queen D2), and have never felt disadvataged by a total lack of a lock. My instinct is that while the ultimate lock strength of the twist lock is not very high, like an axis or lock back, frame lock, or even a well made liner lock, it is very resistant to accidental disengagement.
There are also several other attractive features of Opinels, they are low priced, high performance, public friendly, feel good in the hand, and a good all around blade shape. They are available in a variety of sizes, and are very versatile. When I need a "SHARP" folder, it is often a simple Opinel I reach for.
There are many forumites, myself included, who view the Opinel in the same light as the Swedish Mora knives: Very High performance at a very reasonable cost.
These simple knives put many high end customs to shame in terms of pure cutting performance. While I love my custom knives, you'd be hard pressed to find a custom at any price that will out-perform these simple knives (propery sharpened of course) in terms of pure cutting performance.