My unit has a saggy slider. I agree with others here that there are several variables in the build quality, and mine has always felt a bit loose. I don't think it has gotten any worse over time, the sagginess is just mildly annoying...For instance, when I am lying on my back doing a crossword or playing Angband and using the keyboard. Held normally from a standing or sitting position, the sag is not noticeable.

I am considering replacing the slide with a unit I was pointed to online. Its not critically bad (it still locks in place, but one side tends to "droop" a little...), which is why I haven't pulled the trigger on the change.