You can invest up to £20,000 in your ISA for this tax year in any combination of cash and investments - so a couple can shelter £40,000. If you use your full allowances each year, this will generate a sizeable pot of money that will be kept out of the taxman’s grasp under current legislation.
Any investments held within an ISA are free from capital gains tax when you sell them and there is no income tax to pay on interest or dividend income within your ISA. You don’t even have to declare the gains or income on your tax return.
There are no restrictions on how much and when you can withdraw money from an ISA but you must remember that you only have one annual allowance limit so if you want to pay the money back in you must still be within the overall limit.
If you are a UK taxpayer and you have money to invest, there is little reason not to open an ISA. However, you must appreciate that while the tax rates and tax treatment are currently favourable, the tax treatment of ISAs could change in future.