I now find that the 'circular' polarisers have a linear polariser, to eliminate the glare, followed by a quarter wave plate which produces circular polarisation which can be handled by 'beam splitting' autofocus systems.
So I must find how well my old linear polariser works with the autofocus on my K10 - yes, it seems to be fine. That seems to imply that I can save loadsamoney! Yipee.

So the circularity is in the shape of the filter?
I read it as 'circular polariser', i.e. producing circularly polarised light. That can be achieved with quarter wave plates etc but it seemed not to fill the requirement of eliminating one particular plane of polarisation. The terminology seems to be a bit confusing. I have a rotating, circular shaped, polariser which, I guess is what 'they' mean nowadays.
Not being picky - just genuinely confused as I have a Physics background (not always an advantage).

Good answers. Thanx.
So I'll stick to black, mostly, which is my fave! and not feel bad about it.

So you have this lovely picture and you use it as a desktop, put it in one of those nice Photobooks that Apple do. Everyone says wow nice it is, so you want to frame it.
I bought one of those fancy cutters, which works pretty well, even for me. I mounted quite a few on black card and they look nice (some of them in particular) with the 45degree bevels and all.
But I can't help feeling that I could do better with some alternative colours. Is there a rule of thumb about what colour to choose? It's clearly a matter of taste in the end but a bit of guidance can often save an expensive learning process. I naturally don't want to buy an A1 piece of card for each picture.

What actual effect could I expect from a circular polariser?

"Do you mean at the same ISO?"
Yes - selected manually. Left to its own devices, the camera would upset the 'fairness' of any test.

I imagine that, as the effect of low light get noticed more, there will be a plethora of opinions and folklore about how to get the best out of low light shots. I must say, the effect of noise in darker areas can be quite aesthetically pleasing., although it is sometimes a bit of a surprise.

I hear what you say about 'correct' exposure but there is another issue which the simple audio example doesn't address. That's the issue of what would be called receiver bandwidth, in communications terms. Increasing the exposure time is, effectively, decreasing receiver bandwidth. This improves signal (carrier) to noise ratio pro-rata in signalling applications. However, we aren't dealing with the same sort of signal (i.e. it's a non coherent source of random photons rather than a coherent, weak radio or audio signal) and this will make some difference. I think it is that difference which explains the phenomenon.
However, the same thing applies to film - you get single crystals affected by a stream of individual non-coherent photons and there will be similar statistics involved. The thing about film is that you can't just 'screw up the gain'. You have to use different emulsions. Compensating / exposing for longer in the printing is the equivalent and that will give odd results.
I guess that, in a practical way, you are right and that the ISO number applies to higher light levels and that, indeed, reciprocity does fail at low light levels but the subjective appearance is different. One just has to allow for this.

OK but the problem with that explanation is that it implies that Exposure Time would not be relevant. Reciprocity applies over a huge range of light levels and, when it fails, it must be because each sensor needs a certain rate of photons arriving in addition to the specified number during the exposure time for it to produce a given voltage (which would correspond to the brightness of the scene in that direction.
I guess it implies that the sensor is, effectively, discharging at some, albeit slow, rate which corresponds to long exposure times, whereas the film crystals remain permanently changed, once the light has activated them.

I did a search for this but I could only find comments on fairly ancient threads so I'm asking the question again, in the light of more modern sensors.

I seem to notice that (with my k10D) the noise in dark areas of scenes in poor light seems much worse than the equivalent in bright scenes, despite both having the 'correct' exposure (i.e. longer exposure time for lower lighting). If reciprocity applied, then this should, surely, not be the case. What is going on? It just doesn't seem possible to compensate by using extended exposure times in low light.

That's a very fair comment. It's amazing that, looking through two different lenses, they would both seem crystal clear but, when it's a camera looking through them. . . . . .
What price rosy tinted specs?