Hey Edd, being one who has both bodies, it all depends what I'm shooting as to which body I grab for.
If I'm doing portrait work, or need the higher/tighter grain the 5D provides, then I grab for that one. Like someone else said, they're more for Portrait/Landscape work.
I always grab for the 30D if I'm shooting anything that is fast or I need more length added to my lens. The 30D will do 5 frames per second, and is slow compared to my older 1D that would shoot 8 frames per second. I carry both bodies when covering concerts or events, and keep a wide angle on the 5D and the 70-200 on the 30D. Last week while covering Toby Keith, I was getting pissed off because where I decided to stand in the photographers pit (right up against the stage) he stodd right in front of me the whole time.
When shooting concerts like this, the band managers only allow you to shoot the first 3 songs of the performance. So you basically shoot non-stop for those first 3 songs and try to get as many images as you can. The 5D isnt made for this type of shooting and would freeze up on me after filling the buffer on the card. I think it only will do 17 raw images at a time before it has to pause to write them to the compact flash card. Murphy's Law dictates that while the camera freezes up writing this images is when the pyrotechnics go off and the performer will do something cool that you just missed because you're sitting there waiting for the camera to catch up.
For the average person, this will normally never be a problem, but its something that is making me think I made a mistake by trying out the 5D instead of going with the 1D Mark IIN, which will go up to 10 frames per second, but isnt full frame. Now I've got myself in a pickle, because once you see your lenses in full frame, you never want to go back to cropped.
Another problem I've run into which was something that I never even thought of, was many of my wide angle lenses, a Sigma 17-35 that I used to like alot, now has major issues if used with the 5D. THe edges are always out of focus with this lens on the full frame sensor? My Canon 28-300mm 'L' lens also has major vignetting around the edges. This is something that sometimes can add to the picture, but many times I dont want it to be there and is one more thing I have to remember to watch out for.
So there is one side of having a full frame camera body. Do I like it? Yes I love it, but I hate the slow speed of the computer inside. I hear the new 5D Mark II has moved up on almost all of these numbers, but again, I cant just upgrade every time a new body comes out. So I'll deal with it and look for another used 30D to solve this problem.
Hope that answers some of you questions