I have no issues with circumstance modifiers as long as the intent is to accurately model the situation rather than to artificially bloat skill check DCs in an attempt to present a challenging skill check. If I am going to ignore the enumerated skill DCs completely anyway, I'd prefer to be upfront with my players that I'm going to just assign DCs based upon personal judgement. i'm not really sure where I stand as a player since I haven't actually been a player in a D20 campaign.