The picture above is what came with your TomTom when you bought it, but is NOT a picture of the item you get!
The item you get is a generic suction cup holder that can fit different Sat-Navs, mailed with an extra piece of plastic/adapter so that you can attach the TomTom. Presuming that you get the same thing I did from this seller (a big if) yours will look something like Amazon item # B000V1S3UM (google it).
What makes this suction cup tricky (and may have been the cause of the other reviewers frustration) is that this suction cup has an extra lever to create the suction. With this one you should would wet the plastic, put it on your windscreen, and move the lever. If you don't move the lever, it will probably fall off.
If you order this, move the lever a few times while looking at the suction cup so you understand how it works.
Once it's on your window, it sticks incredibly well. However, don't just pull the thing off - use the lever again to decrease the suction before you remove it!
Besides this being a bit clunky, it's a 5 star product. Because of either dishonesty or laziness on the part of the seller shipping something that doesn't match the picture, they get 4 stars.