Guardsmen are like paper, but their strategy is just the same as the orks... get out as many little guardsmen and keep'em coming out.
If you crap out enough of the little guys, upgrade them to full platoons, and toss in the occasional commanders, you should be able to swarm the other players with sheer manpower. Add in a few of those hit and run walkers to capture/sabotage your opponent's resource gathering and you should be in the lead.
The only faction I have ever had a problem fighting against were the orks... they have the same strategy, only their units are harder to kill.
So in other words most Guardsmen players are in it for the style points than firepower.