FWIW I have compared 5 lenses that I own, on both my camera bodies (20D and 5D):
24-70/2.8L on a 20D @ 50mm: For my style of photography, this lens is disappointing. The lens does not perform at its best wide open, with a bit of softness. F/2.8 at 50mm on a 20D does not provide enough background isolation of the subject. I usually stop this lens down to a minimum of F/4 before I am happy with the results, and at F/4 the background still really comes into play. Where the 24-70L really comes into its own is when you want to include the background and when you don't have time to set up the shot properly. One twist of the zoom ring and you are there.
50/1.4 on a 20D: Excellent value for money. This lens needs to be stopped down to F/2 for acceptable results. Wide open at F/1.4 it is too soft. Colours/contrast are excellent and far and away better than any L zoom lens that I own.
IMO this combination gives a more "honest" rendition of what you saw. The 50mm viewpoint is very easy on the eye but it lacks the drama of a good telephoto.
85/1.2L on a 5D: Absolutely magical. The 85/1.2L is sharp wide open which means that you can really use the F/1.2 (unlike the 50/1.4 which gives poor results wide open). At F/1.2 your images have a distinctive dreamy look which is surreal. By the time you stop it down to F/2.8 this lens will take your breath away by how incredibly sharp everything is.
But really, the only real reason to buy this lens is to allow you to shoot at F/1.2, and I don't like shooting at F/1.2 all the time. The first reason is that all your pictures will look the same. The other reason is: it's a real pain to use at F/1.2. The DOF is so thin, that even if you move a tiny bit during exposure you will throw the focus off. You have to have superb technique to be able to shoot handheld at F/1.2.
Most of my portraits are done with my 50/1.4 or 85/1.2L (but not at F/1.2). I have played with an 85/1.8 and that is what you should get unless you are really moneyed up.
70-200L IS on a 5D @85mm: At 85mm this lens is simply outperformed by the 50/1.4 and the 85/1.2L. I you have either of the primes there is no reason to use this lens at 85mm.
At 200mm it's a different matter though - shoot wide open and you get a pleasingly blurred background. Stop down a little and the background looms over your subject thanks to perspective compression. This is a great lens to use if you want some drama in your pictures, but again, don't over-use this effect.
100/2.8 macro on a 5D: In one word: clinical. I don't understand why the same wrinkle which looks gentle on the 85/1.2L is rendered so sharply on the 100/2.8. This lens is merciless when aimed at people because it is so sharp. If you are going for a hyper-realistic effect then use this lens. Otherwise I don't really like the images it produces because most people would object to seeing their imperfections rendered in such excruciating detail.
Overall I would rank my lenses for portraits as such: 50/1.4, 85/1.2L, 70-200/2.8L IS, 24-70/2.8L.
