Join Date: Jan 2009
Location: Worcestershire, UK
They move fast and often have a 3+ cover save... so are fairly easy to keep away from enemies that'll destroy them in combat, and will often be able to shrug off a lot of firepower.
They're T4(5), meaning that once they get into combat they're going to take much less wounds then standard marines... a sgt with a powerfist bashing a few enemies to bits should give you the win.
They can take lots of special weapons. EG- combi melta, 2 meltaguns and a multimelta... that's quite an anti-tank unit.
They can basically be tooled up to fit any role you want to do: moving 12" and then laying 3 flame templates and dakka'ing with TL boltguns will massacre hoardes, while melta spam can cut through units of tanks, not just one. I wouldn't advise using plasma, since it'll be just as easy for you to die as for a normal marine, but it is still a fun option (I like plasma... I know its not great anymore but just don't care).
They can easily be made troops... so you have a nails unit that can crush the enemy, then win the game. A captain on bike is pretty hard, especially if you then also use a command squad with a few power weapons and an apothecary... very scary.
The downsides of them are that they can get a little pricey (though not too bad) and they can't hide in metal boxes. This last point is important since it means anti-MEQ weapons are much nastier against you. Things like battlecannons or plasma will erase them very rapidly, especially if you didn't turbo-boost that turn (such as first turn of the game).