We always have vouchers knocking around too if anybody wants one.
If you are going to use one here's my 2p: spend it as quickly as you possibly can.
Why - no, not because it may not be there tomorrow, but because it will give you a better feel for how much of a difference it can make to your business. £50 spent in an hour, provided it is on keywords that are genuinely relevant to your business ("golf" is not relevant to someone promoting a golf course) will produce you the same amount as spending over two weeks. However it will be much more difficult to detect the difference it makes to your business over that time.
An additional 2p.
Ask yourself if £50 is enough to make a decision on whether AdWords works for you. There are very few businesses where this is enough. (Happy to explain this if you want!)_
__________________ Tim Barlow Attacat - Specialists in Google AdWords and other paid search marketing
Keep in mind though that these vouchers are usually only available for new Adwords users. Unfortunately, you can't apply them to an existing account (at least, that was the case the last time I tried, a few months ago).