I've got the 70-200 F4 non IS, bought as my first L lens. Light, stunning quality, excellent lens.
At Christmas I bought the 70-200 F2.8 IS and it's a step above for what I shoot, which is mostly sport with it. The f2.8 means I can easily isolate players, and get extra shutter speed, the bokeh is beautiful and it's just a great lens.
But it's all down to money really. For 1/3 the price, the F4 is a great buy.