Originally Posted by GeneV 
Not only does it add an extra step, which you have to remember, but the results are often inconsistent.
Note the inconsistency seems unique to the K10D and perhaps K20D - none of the other Pentax cameras seems to have any problems metering M lenses at any aperture. And also, supposedly, only an issue with the stock focus screen. Not sure which model the OP has.
As for the "extra step" and having to remember it, this ceases to be an issue if you start shooting in M mode all the time. I made that switch partially for this very reason a year or so ago, and I think it has improved my photography markedly. Not that it is impossible to get the same results in another mode, but I find it models very nicely the way one should be thinking anyhow in order to get get consistent exposure. I find that overall, there is *less* button pushing and *less* to remember shooting in "M" mode and leaving exposure fixed based on ambient light than shooting in Av mode and trying to figure out how much exposure compensation might be required on a shot-by-shot basis because of the local values contained in that scene. So I personally will be sticking to "M" mode even if I strike it rich and decide to replace all my "M" lenses someday with auto-exposure ones.
Anyhow, if I had a collection of "A" lenses, I'd keep using them (but in "M" mode) as long as I could, whether that was months, years, or decades (my money's on the latter...), then replace them as necessary with whatever seems to make the most sense at the time. No reason to replace a lens that is working fine!