I am a final year Computer Science student, and I have just come back from my placement year. It was definitely worth doing - you get experience in industry, which will help getting a job at the end, and you get some money.
We did have to pay 50% tuition fees (~£600), even though we didn't have any lectures. When asked about it, the placements tutor said that it is to cover the cost of running the placement scheme.
With regards to grades, last year the only people that got 1st's were people that did a placement, and the only people that got 3rds didn't do a placement. They reckon that doing a placement increases your final result by up to 1 classification (i.e. you could get a 2:1 instead of a 2:2 if you do a placement) - this is because you learn time management skills etc. which are very important in your final year.
Overall, I would say go for a placement - its a chance you wont get again.