Will the slip make you size smaller?  No, it won't, but it will discipline your body and control the folds of the skin.
Will it make you slimmer?  No, but you'll have smoother lines and as a resut tighter clothes will look and fit better. You might find out that a dress you considered too tight will prove ok :)
Will it roll up?  Just a tiny bit and just in the lower part below waistline. The top part sits where it should be - I've tested it myself and it hasn't let me down.
Do you need hold-ups with it?  No, but you will have to put on tights first if you decide to wear them and then the slip.
Is it difficult to do up?  Not any more than any body.
Is it difficult to put on?  YES! You put it on like briefs, through the neckline, the brief part should be done up. Pulling it through the hips is the most difficult part, but after that - satisfaction guaranteed!
If you hear thread cracking it does not mean the slip is going into pieces - it will just be "supportive" stitches - we try to make them crack before sale, but not all of them crack without putting the slip on and that's why some of them will crack during your trying on.
To sum up:
The slip exposes the bust, flattens the belly and shapes the waistline, smooths the hips and back, rounds the bottom and brings all the skinfolds into control :)

What size should you choose? - I suggest going for the same size you buy effuniak briefs or size down. Do not try down to size down by 2 or more sizes, as instead of hiding skinfolds you will highlight them even more. The elastic fabric of the slip will cut into every fold when it is too tight and the effect will be the exact opposite of what the slip was made for.

NOTICE!

The bra visible in the photos is not a part of the slip and is sold separately.