It really is a matter of personal choice
Many people prefer to hand write their letters as they feel it is more personal, others routinely type their letters.
I always type and laser print my letters as I just don't have the time to handwrite all my letters as I am collecting for charity. My letters also usually go out on the charities own headed paper
To be absolutely honest I don't actually think it matters much either way, and thus does not have much effect on success rates
All I would say though if you type your letters, is that it is important that your letter does not look like a "form letter" that is to say just a generic mail-merged letter. Try to make them as varied as possible
Ian
