The 5D is a great camera. I've been using one for a while now and just added a 7D to our toolkit. The 7 handles better but at a great cost in image quality. My wife and I just about always reach for the 5D first.
A few 5D tips I've learned from using mine:
1) I hate the Adobe Standard preset in ACR/Lightroom for the 5D. I almost always pic one of the "Camera" settings as a starting point - usually camera neutral. When I first got it and used the default Adobe Standard I was really disappointed with the camera.
2) The video quality is amazing, but it's also really challenging. It's much better with the firmware updates they've made. While I usually use Av mode for photography, I just about always use M for video. Stick with a shutter speed that is double your frame rate. You can go as low as equal to your framerate in low light, but don't go slower and don't go faster. You'll want either a variable ND filter or a set of ND filters if you want to shoot a lot of video in outdoor settings so that you can open up your aperture. I also use a custom preset with the sharpness and contrast turned all the way down and the saturation dropped several steps. I get better results adding sharpness, contrast, and saturation in post production rather than letting the camera take care of it.
If you get really serious about video with it, read the forum at dvinfo.net and consider putting the Magic Lantern firmware on it.
3) Shoot wider than you are used to. It's got a lot of pixels and they are wonderful pixels. By shooting wider, you have more options in cropping. It's much easier to use an image that fills a 1.78 aspect ratio HD TV and works for a 1.25 aspect ratio (8x10) print.
4) The AF is its weakest feature. It's reasonably quick and accurate. The problem is that it covers so little of the image area. I'm a huge proponent of moving AF-on from the shutter button to the back of the camera. That's even more important with the 5D because I do a lot more focus and recompose shooting.
5) I stay out of the high expanded ISO modes. I get results that are just as good by sticking with ISO 6400 and using exposure compensation to simulate ISO 12800 and 25600. The advantage to using ISO 6400 is that I can prevent blowing highlights if I decide that I don't really want that full extra stop. I do use ISO 50. It doesn't seem to be noticeably better or worse than ISO 100, but it's handy when you want that extra stop of shutter speed and don't want to whip out an ND filter.
6) Many people dis the resolution on the 5DMII saying that it is unnecessarily large. For most snapshots, I'm sure that's true. Right now, though, I'm working on some studio shots I took with the 5D. Using good lenses at f8, the detail is stunning. I'm making 16"x20" and using every bit of that resolution. On the other hand, it's pretty easy to loose the resolution advantage with less than perfect focus, camera movement, or even pushing your lenses too hard.
7) You want big memory cards. The RAW files are big (20 meg) and HD video eats up space (about 3 minutes per gig). I use 32 gig cards in mine and find that they are pretty comfortable. You could easily get by with a 16 gig card if you don't do much video. I wouldn't walk around with less than that unless you are bringing multiple cards.
8) I don't like the highlight priority mode. It disables ISO 100, which I don't like losing. I also don't like the image effect. When I'm worried about highlights (like my son's soccer teams stupid white uniforms - try shooting an African American kid in a bright white shirt in harsh sunlight

), I usually just dial in -1/3 or -2/3 stops of exposure compensation and adjust in post. It's got so much dynamic range and such low noise levels at low ISOs, that this isn't much of a sacrifice for non-demanding material.
9) If this is your first FF digital, it'll take a little while to adjust. Obviously, you lose reach on the long end and gain width on the wide end. You also get roughly one stop narrower DOF and about one stop less noise. You'll also start to see edge defects in lenses that seemed great before. In particular, some lenses or lens and filter combinations that were fine in the past will now vignette.
10) The batteries are insanely expensive. Fortunately, they hold a good charge and have great remaining charge metering. We bought one spare that we share between the 7D and the 5D. You generally won't use a full battery in a day unless it's an unusual day. I've done it shooting a golf tournament. I'll probably do it at a wedding next weekend. I don't think I ever did it at WDW, but I made sure that I had a full charge every day.