Computer mice are one of those items that almost everyone needs these days, and they also have an extended life. By branding computer mice with your companies name and/or logo, you are not only providing people with something they need, and will use, you are also going to get more promotional push for your money than you would with something less sturdy, and more disposable, such as a pen or pencil.
While pens, pencils, bags, and similar items give you a lot more coverage than a branded mouse, they are also inexpensive, readily available, and have a limited shelf life. These promotional items are great for timed campaigns - such as grand openings, or when your company is running a limited-time special - because they will get into a lot of peoples' hands in a very short time, but they will also be in their hands for a very short time. An imprinted mouse, on the other hand, could feasibly last a year or more!
This benefits you in two ways: First of all, the owner appreciates having something practical he/she can put to use, especially something that is going to last him/her a while. Not needing to pay 10-20 euros or more for a new mouse for a long time, even if they don't start using yours right away. Secondly, your organisation is being promoted whenever that mouse is put into use - for as long as it is used. Again, that is going to be several months, at least - even longer.
They come in many shapes, sizes, and models, including mini-sized and USB - some can even be carried on a keyring, or in a pocket. These are handy devices which, once emblazoned with your logo and/or name, become extended promo products that are easy to carry, and practical for the office, at home, or with a laptop. Many attractive models can be had for under 20 euros, and some are available for under 10 euros.
pencils, notepads, and similar products are great for coverage, they
get that coverage specifically because they are disposable. Sure,
everyone needs one from time to time, but they are readily available and
inexpensive, so no one worries about losing or replacing them. If you
really want to dazzle potential customers, and partners alike, and get a
lot more mileage out of your advertising dollar, give people something
practical they will hold on to, like imprinted umbrellas, or printed computer mice.