My marine army is, surprisingly on foot, and it does quite well. And comes with three dreads as the only vehicles.
Thing is a long time ago I discovered my scouts were very lucky. Then I realised how awesome a squad of five scouts with shotguns, led by a sergeant with a plasma pistol and an powerfist is. That sergeant has killed many enemy commanders armed with p.fists and thunder hammers, because of a)instant death and b) simultaeneous combat. The scouts around him sometimes kill things in combat too! Especially units of Terminators. Statistically I think the squad on a shoot and assault turn can kill 3-4 termies, more than making up for there cost. Lovely. And some might even survive (not likely though).
I take two of these units of scouts. Plus with combat squads there's another five just with bolters and another five with sniper rifles and a missile launcher hanging around the battlefield. Maybe a unit of tactical marines, definitely a unit of devastators or two, a badass HQ with a tooled up command squad wandering around that surprisingly has never really been targeted. Its basically a marine horde army designed for lots of short ranged firefights and a few select assaults and long range firefights.
And in the middle of all that three dreadnoughts, slowly walking forward doing very well as all-rounders. Because the emeny just had his thunder-hammer wielding commander killed by a scout sergeant and he's being swamped in a few places the dreads, walking forward somewhere, firing here and there, taking out marines in there ones-and-twos each turn means they get overlooked. Occassionally I get them locked in combat to tie up an enemy unit which hasnt got any way to deal with the dreadnought, but they do better wandering around swiping at enemy vehicles and have alot of fun fighting enemy dreadnoughts. One was one stuck in combat with an enemy dread when they were both immobilised.
So thats how I run my dreads, if it were a more normal marine army Im sure they would get blasted pretty early in the game.