I used steel rain and 'The Boss' for the first time during the general assault scenario last night and have to say that they were a lot of fun.
Steel rain was very useful without being broken, but I can well understand why artillery does not ignore cover. The number of dice I was throwing during a sustained, 2-rocket salvo were phenomenal!
Boss was useful. Bringing my BBQ squad back on, (after about 2 tries!) was very helpful in making my opponent Scott go all wibbly with dread! The mechanic kept Steel Rain in the game, while the walker sheltered the command squad in turn.
If you haven't got them yet, then the command squads definitely make for a longer, more interesting game without breaking it.