Thanks for all the responses. I did like the 100-400 as previously mentioned, but I found myself like many, going to 100 or 400.
I have read opinions on the lack of IS and have found mixed reviews. On one end, its 400mm, where's the IS, hello!! and on the other end, most cameras these days, are capable of great results at ISO 1600, 3200 and higher, so shooting handheld at 1/1000 or with a monopod at 1/400 isn't too far fetched.
I have also heard that this lens is corner to corner sharp at wide open (5.6), which helps.
I have also tried the Sigma 120-400mm OS which had fair reviews, but I found it to be (my copy at least) not sharp enough, and soft, especially at 400mm.
I might pull the trigger on the 400mm 5.6, the good thing about buying gear is that most of it holds value pretty well. I can buy something in good used condition (i.e. $925 for a UZ copy of the 400 5.6, box and all), shoot around with it for a month or two and resell it and will most likely break even, or even make a few $$.