There are some caveats in using the polarizer. It can be really useful at times but I would not advise to keep it on your main lens at all times. The main reason for this is that it will make the use of a tripod compulsory with slow film or low light conditions. There are specific cases when you should not use a polarizer.
Situations where the use of a polarizer is not desirable
The polarizer does not make the clouds "stand out" in uniformly overcast weather. All it does is turn them a little darker. You still lose 1.5 stops of light, though.
In the evening, the skies naturally turn darker. It would be tempting to keep the polarizer on your lens to get dark blue skies, but the risk is to overdo the effect and get unnatural things.
Honestly, I have never really tried to use a polarizer at night. Apart from potentially ruining side effects, I would be concerned by the fact that the filter tends to turn the viewfinder darker. As I generally focus manually at night, it would make it even more difficult.
Other potential gotchas
Here are some situations where you should be careful in using a polarizing filter.
White or clear buildings
The metering system of most modern SLRs is calibrated to take a medium gray as a reference for metering. If you take a reading from a lighter area - typically a building occupying most of the image - you are likely to underexpose your shot. With a polarizing filter, it will be even worse. You will get really dark - almost black - skies.
This is more likely to happen with slide film as digital pictures have less contrast. In that case, take the filter off or turn it around so it has no effect and overexpose a bit. Use bracketing to make sure you have at least one acceptable image.
When shooting outside with an ultra wide lens, you are likely to cover a wider sky area and will certainly get both darker and lighter zones in the sky, depending on your orientation.You will also have a difference between the area close to the horizon and the top of your image in vertical views, because of haze.
These differences are amplified by the polarizer and can get distracting.
Be careful when choosing a filter to fit on a wide angle, especially on a zoom. At the wider angles, there can be a visible light falloff on the edges. To avoid this, get a polarizer specially designed for wide angles. Singh Ray makes a wide angle polarizer and Hoya has a range of "slim" filters. Be aware that the price for these will be all but slim!
More polarizers... and links
Some makes propose filters that do more than usual polarizers. Some add an UV filter to the polarizing effect, others a color filter. I have not tried them yet, but the warming polarizer looks tempting. You will find information on the following websites:
- Thkphoto proposes Hoya filters (if you find an official Hoya website, please let me know!)
Singh Ray has a whole range of polarizing filters including
- Gold n Blue polarizer
- Warming polarizer
- Red-Ray™ Polarizer
- Cokin proposes circular polarizers in addition to its famous range of P and A mount filters
- Some camera brands such as Canon also have a range of polarizers.
Here are other articles about polarizers