I bought this item as I didn't want (nor was able!) to spend a lot of money on maternity clothes that I would wear for only a few months.
In theory this is a good idea. At 34 weeks I am still using the first belt - there is a second larger belt, and in turn these can be combined to give an even longer belt, so it will definitely last for the duration of my pregnancy. However, the buttons (two per belt) are in a fixed position so most of the time you have to choose between too tight (not recommended of course) or just that bit too loose, with the result that I spend a great deal of time hauling my jeans back up as they gradually fall down while walking around! Yes they are on an elasticated band, but it doesn't seem to hold all that well.
I did find that even with the smaller belt I need to use the fabric insert which ends up being bunched up since it isn't designed to be used with this size of belt. Also, although it will prevent you from flashing your underwear, it's still perfectly obvious you're going around with your zip down so you'll need to make sure you have some extra-long tops to keep covered up. Also, on this size belt, one of the buttons has been sewn onto the elasticated section and I found that after a while it did fall off and had to be resewn.
Overall I would still say it's a good idea, but the problems mentioned above mean I do tend to only use it when going out, and stick to tracksuit bottoms at home.