i own the 2.8IS but i dun like it - heavy
i played 4 and 4 IS and dun like both - f4 and not so smooth bokeh than 2.8.
2.8 non IS okay but dun like too - need slightly faster shutter speed for non-steady hands/arm
if i only had one lens to use on work... i'll bring the 2.8 IS... certainly weight is an issue (even with a 5dmk2+grip) dont mention 1'series... but the 2.8 far outweighs the F4 is terms of handiness and weight factor can be avoided by either getting a weight reduction strap (works for me) and a pulleybag... which i just drag everywhere i go...
as for the IS... i feel it's important for me too... anything below 160 and i'm worried.. so IS is a godsend (not a very stable person i guess)