Here in the Lomo world, you hear about expired film A LOT, and it is something we all "want to or have tried". The ones who haven't tried it can be afraid of buying something that supposedly doesn't work anymore ... but is it true that expired film doesn't work anymore? I don't think so!
First, I'll tell you why the film manufacturers put an expiration date on the film and for that, I have to explain to you the nature of the emulsion.
When a film stock is produced (which can contain many film rolls) it is made to achieve certain sensitivity (or speed), a.k.a. ISO (or ASA); it can be 400 ISO or 100 ISO or 800 ISO. Then it gets out of the factory and sent to the store for people to buy it. There's no way for you as a consumer to know how the film was transported or stored before it arrives at your doorstep, and depending on conditions (temperature, lighting, humidity) the film can lose a certain amount of sensitivity. That's why you can sometimes buy a film rated 800 ISO but in reality, its sensitivity is 400 ISO. This can be a loss of one stop, and sometimes you don't even notice it.
Now, the date the manufacturers use as the expiration date is the one they can assure the film to not have lost a lot of its sensitivity and in a way to still be the one printed in the package. After the day the film expires, the manufacturer is not responsible for the problems you may encounter when using it, but the emulsion still works properly. The chemicals on the films' emulsion would eventually lose light sensitivity, however gradually!
So, there's no problem when using expired film and the only thing you have to take into consideration is that you'll need more light than stated. A general rule is to shoot the film one stop slower than box speed for every ten years past the printed expiration date. Assuming that the film has been kept in ideal conditions as per manufacturer's recommendations. For example, if you buy a roll of film that expired ten years ago and it is supposed to be 400 ISO, maybe you should shoot it as 200 ISO. Knowing that in advance, you won't have problems using the insanely fun expired film and the results will be great.
And what are the effects on the image when using expired film? It's hard to tell! you could get more contrast and intense colors or the complete opposite. Also, the colors can shift a little bit. For example, light blue could get a bit purple; but this isn't always true, they can change in different ways or they can stay the same. This is what makes expired film so exciting, the results may vary from one to another!