Well my first question is what armies do you face the most??? you mentioned nilla marines,
Me, I love my assault marines, but you have to play them right.
Use cover and woods to your advantage(stand behind woods, not in them) If your opponents knows you will have assault marines he will probably place a lot of area terrain to make you take dangerous terrain tests for landing in it. But remember if you choose to walk you don't take the test, and i believe you don't test for jumping OUT of terrain either.
Second, If you are playing objectives(and 2/3 of the time you will be) assault marines are distractions, if your opponent is shooting at them he isn't shooting at your other units, and if he doesn't shoot them he is going to have ten space marines dishing out 40
Str 4 attacks against his battleline. Plus never EVER underestimate bolt pistols, great for forcing saves on enemy elite and
HQ units.
I run my assault squads ten man strong, 2 plasma pistols(These do NOT suck) and the sergeant has a Storm shield, melta-bombs, and either a power weapon or fist. They are sometimes accompanied by a Jpack chaplain.
This build i have found effective against most of my opponents, they are especially good against T'au, because if even one marine gets through the T'au will fold like an omelet. once a tau gunline whiped out my squad except for the sergeant who has a
SS and Pwr sword, that sergeant killed a Shas'o, 2 suits, 12 firewarriors(due to
SA) and 2 stealth suits before the last stealth suit mugged him from behind and he failed a save. Final tally Assault squad cost 250 points, Shas'o 102 pts, bodyguard 106 pts, firewarriors 150 pts, Stealthsuits 60 pts. My 250 pts loss vs 418 T'au and the game was an objective mission