Selenium cells take a time to die, and the linearity of response seems to go first. I was having periodic underexposure problems and eventually traced it back to my Weston V in the mid to low EV range. Spot checks in good light failed to show up the discrepancy.
I have meters from the 1930s (literally) that are still active and lively. I've got selenium meters that are dead. A lot of it depends on how well they were kept. Humidity kills cells. Dirt and grit foul meter movements. Dropping them doesn't do them any good at all.
It's a good idea to check any meter against a known-good one for accuracy across the range you expect to use it. Any meter, expensive, cheap, old, new, can be off.
I am still using a Weston Master II from around the late 1950's. It still seems to be working when checked on the Kyoritsu and the good old "1/film speed = seconds at f 16" guide line in daylight. I even bought a Weston Master V to be sent out for repair to go along with it. I like them. They do not require batteries, even if they go not go down very far in low light. If I really want take a photograph in moonlight, I will drag out the Gossen Luna-Six or something equivalent.
If you keep the Weston meter in a dry environment, it should last a very long time and give you very good service, except for the extreme low light level thing. Low light level; OK. Extreme low light level; no.
I still believe in older technology. Just because something has an excessive accumulation of years does not mean that it is obsolete. (I hope that statement applies to me also.)
I've had very good luck with selenium meters, although I do have a Olympus Auto Eye where the cell has definately degraded over time. Nicholas' explanation of the combination of light and moisture degrading the cells is the best and most logical I've seen. It also explains why meters (kept in the dark most of the time) hold up better than cameras (exposed to light and moisture much more). I have had more problems with CdS meters than selenium.
I must report that my trusty Soviet-built Leningrad IV selenium cell meter has worked fine since I purchased it in Czechoslovakia, for 14 bucks equivalent, in 1978. All remarks about selenium cell meters at very low light obtain for this one, as well. BTW, despite its provenance, I have detected no excessive red sensitivity. Nyuck, Nyuck.
You can be lucky and they keep working, or like in my case the Weston Master V cell gave up. Many Weston's sold second hand have cells that are on there way out.
I use a Leica meter, which I want to replace, the Leica dealer I use won't sell the ones with selenium cells at all because hesays they are nowhere near accurate. Mine is but it's damaged and temperamental.
You run the risk of not noticing your meters failing so it's important to check it every few months, (about twice a year).
My experience mirrors what others have posted. About 1/2 of my selenium meters are shot. The other 1/2 work fine.
In case of need, this company offers custom cut, contacted selenium cells:
(I posted this here a year ago; perhaps someone has given this a try by now.)
Interesting. I've seen the Megatron site but for some reason I never really looked at it.
"Our experienced exposure meter specialists also recommend a wide range of repairs and servicing to all Weston Master & Euromaster exposure meters: Calibration check - £22; Recalibration, servicing and minor adjustments - £36; New photocell fitted, recalibration & servicing - £42; New movement fitted, recalibration & servicing - £55; New photocell & new movement fitted, recalibration & servicing - £66. All prices include UK carriage and VAT."
That's a lot of money, especially at current exchange rates. But if you love that Weston Master, that might be worthwhile.
That special service is new to me. I rather thought of ordering just a cell of the size needed to substitute for a died cell. Especially for in-camera meters.