The advantage of cash ISA's is that the interest is paid gross (ie you dont pay tax on it). The disadvantage is that you can only put a maximum of £3k in during any one tax year, and once withdrawn cant be put back in again if this would result in you exceeding the £3k.
If you are a taxpayer than I would recomend making as much use of cash ISA's as you can.
Dave
P.S. I feel obliged to point out, I am speaking from experience and my own personal views above - I am not an advisor, nor an expert.