I own a Böker Speedlock. It´s mechanism is reliable. It has a helical spring to open the knife. That spring is not prone to breaking and it will open the knife completely even when the blade is stopped before locking.
The knife can be closed with one hand a it can be disassembled for cleaning (it´s pretty easy to reassemble it too <img src="images/graemlins/wink.gif" alt="" />). Like most switchblade knifes it´s intended for righthanded use. I can use it with my left hand too.
I can recommend this knife. It´s one of my favorites.
If you´re going to buy an assisted opening knife you should stay away from the ones with a flat spring in the back (prone to breaking and the spring can lose power when you store the knife closed) and those that push the blade out of the front (they don´t lock reliably)