It is cheaper than the "standard" 17p/MB from other providers and lasts 30 days so you do not have to renew everyday. Three's europass is £5 and has unlimited internet I think, but only lasts until 12 midnight.
So it is cheaper than the 17p/MB but not that cheap still. You'd still need to use it with caution.
The best idea would be to allow you to carry your allowance abroad.