Marbo is perhaps one of the coolest guard units out there. In an army were mass attacks are everything, his one man approach really sets him apart. He is infamous for his ability to pop up, drop a big blast and sacrifice himself for that one perfect strike.
Marbo pops up by a ruined window...
...right below a Blood Angel Objective, on the last turn of the game.
That said, Marbo is a lot more useful than that. His ability to simply show up Anywhere on the table is not to be sniffed at. In fifth edition I have used this to my advantage, drawing rage inflicted combat units away from my gun line. In sixth this has become a valuable line breaker tactic when you desperately need those extra secondary objectives. And he has always been great at objective denial, particularly if he rolls in too late!
He is also simply dirt cheap. For just over the cost of a standard infantry squad with a heavy weapon, he adds a great degree of flexibility to the guard force. Its now extremely likely that he will arrive turn 2 or 3, so your opponent is left chasing his tail, maybe playing more defensively for fear of that large blast template.
We also cant forget his rather amazing combat stats....for a guardsman. I have used him to break combat deadlocks, particularly when blobs of guardsmen are engaged with light armoured troops (such as Ork boys or even other guard). He will never be a Power house in close combat, and I certainly wouldn't send him in unsupported, but he is great at taking challenges against squad leaders (Ork nobz, commissars etc..)
So the down sides...
He is still just a guardsman, with his lowly toughness three and 5+ armour. But with his stealth ability he can often get 3+ or even 2+ armour saves.
His ripper pistol is certainly not going to win any games, and whilst he has a great BS I would never depend on his blast template 100%.
In this way Marbo is more of a means to cause distraction, forcing an enemy to second guess his own battle plans, whilst giving you a small degree of strategic flexibility. Because of his fragile nature and his relatively cheap costs, you can afford to gamble with Marbo. However this is were it is important never to rely on that gamble. He his potentially game changing, but rarely game winning.
So what do you think? Is he just a human bomb? Or does he have the potential to change a battlefield?