I have the 70-200 f2.8 Non-IS.
It's a fantastic lens, but here is my take:
Neither of the non-IS versions are weather sealed. But then neither is the 550D. Is this a biggie? It's not for me. I use a kata rain slicker when it's bad anyway (for my 550D and 7D)
Although you have the extra stop in terms of aperture exposure, I find the lens is at it's best from f4, especially towards the 200mm end. F2.8 is fine for video, I wouldn't pixel peep my stills at f2.8. Not that I pixel peep anyway. You have to weight this up against whether you use your current lens wide open or not. You may for example use your current lens at f5.6, I don't know.
The 70-200 f2.8 non-IS comes with a tripod collar. I generally use this lens on a tripod or monopod. It makes panning so much more easier, and goes some way to negating the IS feature of the alternatives.
One big performance boon is the effect of the max aperture on AF.
Even if you are shooting at f4 or f5.6 or whatever, the lens is wide open until you depress the shutter button.
That extra stop really brings Canon's AF to life. Now the 550D is quite basic AF, but you do have that centre cross AF point, which is extra sensitive with f2.8 lenses or faster.
If you were shooting sports or nature, and using AiServo, then that f2.8 could make a hell of a lot of difference to you. I used to use a 400D for shooting jetskis and speedway motorcycling. With the centre spot selected in AiServo mode I actually got some brilliantly sharp shots. The 550D is never going to compete with a 7D or 1D for AF performance, but if you want to get the most out of what you've got, that extra stop makes the difference.
In short I am really happy with my 70-200 f2.8L. If you shoot video, or sports etc then I think it is worth a punt. I know loads of folks rave about their f4's and particularly their f4 IS's, but I really wanted the AF benefit and extra aperture stop.
I wouldn't make this an issue about shutter speed, because higher ISO's are actually good if post-processed properly with the digic IV cameras, and besides, IS does not counteract subject motion blur, which is actually the real killer.
I would say that if you are looking at any of these lenses then get a monopod. It makes all the difference, not just for stability but panning.
