In the past I've worked under Giant and Parasol. I used Giant back in 2008 when I wasn't sure if I was staying in contracting for forever. All umbrellas are much of a muchness; there's a legal limit on how much tax efficiency they can help you with, so you'll take home the same with whoever you choose. In the long term a limited company is definitely the way to go.
They're just a middle-man who invoices on your behalf and pays you based on that. Using someone like Giant you're ceding a lot of the financial (tax) advantages of contracting though, since you pay tax as a PAYE employee.
Those tax advantages will start to disappear next year.
Obviously it depends on how much you earn and pay yourself, but I think for most people they'll still be better off as a limited. Next year's changes only add up to about 2-3% increase in tax for someone taking minimal salary and the rest as dividends.
People I know who've worked with them are pretty happy. They've got a reputation for being super super responsive, too, so email or ring them and ask?