I used a sweat suit last year when I did SW last year to aid my home exercises. It is really good (its what boxers use to make you sweat more). You mainly lose water becuase it makes you sweat buckets. the cuffs of the sleeves and legging are tight elastic which prevents the sweat from running out.
You sweat buckets, so you did need to make sure that you hydrate more regularly doing exercise. It certainly isnt something I would wear outside of the house becuase it is quite noisy!
A cheap alternative to get an idea of the effect is to use bin bags wrapped around your body, tied at the wrist and ankle.