I've played my mate 3 times in total now - and he uses death guard.
Of the 3 games, first I used Astra Militarum, a mix of guard and scions, was completely un-prepared, having forgotten his marines are T5 and because we mis-played some rules, I got completely curb stomped.
Second game I was more prepared (see: spammed plasma) and made a pretty convincing win - although I did loose some guys and ultimately it came down to the objective not appearing until really late game, and due to shaken he was unable to get enough models to secure it.
Third game I played deathwatch, and curb stomped him - having tabled him by end of turn 4 and only losing one guy myself. Admittedly he had some really abysmal rolling to blame for my lack of casualties.
Ironically, in that game my only casualty was to his poxwalkers.
My general consensus from this is:
If you bring many pox-walkers - make sure they clump. It's the only way to counter their LD4 when you inevitably have to take a few nerve tests.
His issue vs my deathwatch (disregarding the fact he could not hit me) was lack of higher strength weaponry.
Your T5 and disgustingly resilient is massive.
The pox walker strategem is also big, ensure you have a point for that if you expect to be in combat with them.
I think he may have been better off bringing a few fewer poxwalkers for some weaponry upgrades (although he was trying to play
WYSIWYG with the starter set marines)
Bonus: Bring dice that are rigged to roll 6's for your disgustingly resilient saves, you will annoy the hell out of your opponent
It does also depend who you're fighting.
If you're playing against T3 models, and can get into combat, lots of poxwalkers can be good, anything higher and you might find they're a liability.
Also, don't go near flamers, the roast your nasty poxwalkers.
His list was:
Deathguard
10 models total
Leader:
Plague Champion
Powerfist
Bolter
Plague Knife
Zealot Specialist:
Plague Marine Figher
Bolter
Plague Knife
Combat Specialist:
Plague Marine
Bolter
Plague Knife
Icon of Despair
Demolitions Specialist:
Plague Marine
Bolter
Plague Knife
Non Specialists
Plague Marine w/ Plasma Gun
5 x Poxwalkers