I have a Katzeye split-prism for my 7D, and I had a cheap-o $15-ish one from ebay for my 400D/xti. The cheap-o one had a dual-diagonal split-prism, which worked very nicely, while the Katzeye has a horizontal one, which is often hard to see.. I also have a KPS 1.3x multiplier eyepiece thing, and if you ask me, it helps with focus-screen accuracy 10 fold. I go from eye-strain without it to super easy to know what's actually in focus with it. I would highly recommend one.
I actually prefer the microprism part of the focusing screen, so if I could pick up a $35 EG-S screen for the 7D I would in a heart beat. I imagine I would like it. And $35 is not much of an investment (and if I hated it, I'd just sell it again..) Never used one, though, so I can't tell you the difference.
My Katzeye is *not* "optimized" for brightness (the OptiBright thing), by the way, so 100% of the screen actually shallow depth of field. I can use outside the microprism area to focus, but it does get dim out in that area in low light.
Oh right.. As for your f/4 lens, I'd agree and say that from my experience with my screens, f/4 is OK, as long as you've got decent light. f/5.6 is do-able, but borderline frustrating.. If I could only work with that outer part of my screen, I probably would be unhappy at f/4, but again, no idea how it compares to the EG-S. I imagine if the 50D is OK at f/4 for you, then the 5D should be just as fine, or maybe be a bit better, since you technically have more light getting to your eye, due to the larger prism? (Feel free to tell me I'm wrong there..)