I run a marine bike army and have good success with it.
The way I look at it is like this. For 7 more points over a regular marine, the biker as a troop gets
*
TL bolter
* Relentless
* T4(5)
* The ability to turbo-boost
* 12" normal move
* The ability to take 2 special weapons per squad
Just like running an army with all
TAC squads, an army of just bikes/speeders will not be super-competitive. If you want to really do well, you will need to add long-range anti-armor to it (such as rifleman dreads)
That being said, its fun to play with ~30 bikes in friendly games. You can cover the board with them and their great fun to play with or against.
In the following thread, we talked about bike army lists.
http://www.dakkadakka.com/dakkaforum/posts/list/353194.page