I never played a game of 40k with my Space Marine army without taking at least one landspeeder, and often more (I field up to 6 in game).
They are the most fragile part of a SM army, but potentially the most powerful. This was finally illustrated in the most recent WD, and it was the first time that a SM player in a WD battle report didn't simply use their landspeeders as a fast moving Tactical squad.
Landspeeders are perfect for setting up crossfires and capturing objectives and/or securing quarters. More importantly, their heavy bolters are awesome for ripping into ANY infantry unit, or if they're armed with multi-meltas...expect a lot of dead tanks.
I can't tell you how many times my landspeeders have been the unit(s) of the game -- and often to the surprise of my opponent. I will, however, admit that they are not as forgiving as a Land Raider or Tacitcal Squad. If you leave them in the open, if you don't keep them moving, or give them the right kind of support, they will be blown out of the sky rapidly by infantry with luck (a single bolter hit will often do the trick) or heavy weapons with ease.