i know a few folks who have done this, and mixed results...
one friend weighed in around 400lbs, and at 6'2" he was quite a hefty guy... he's now down to 210lbs and no longer diabetic and enjoys jogging 5 miles a day... enjoying a healthy life...
another friend had it done, she weighed over 350 and only 5'8" but has not lost what she expected to, although much lighter now (not sure of the loss) - definitely happier though
i wish my dad could do this, he's over 300lbs but only 5'9" and diabetic, amongst other cardio issues, he also has rhumatoid arthritis which adds to joint pains, etc from the added weight - but unfortunately with the meds for his symptoms he can't go under the knife - i know it would add 10-15 years to his life...
