After 5 years of digital EOS photography, I think I've settled on my ultimate line-up.
As the title says, not quite the holy trinity but close enough, and with a couple of extras..
Here's my progression:
300D -> 5D -> 1DsMkII
Lenses (all now gone):
18-55, Tamron 28-200, 50 1.8, Sigma 10-20, 70-200f4L IS
In my opinion, the 15 and 85 are almost as good as the L's.
I found that I wasn't using the longer length of the 70-200, and I can put my 1.4 TC on the 135L on the rare occasions I need extra reach.
The wealth of information on here has helped me to get to this setup, and there's nothing I'd like to change now.
Lessons learned along the way:
1. Just because everyone on here raves about a particular lens, doesn't mean it is right for you. True the 70-200 4L IS is amazingly sharp, but it rarely went on the camera because it was too long for most of my shooting, and when travelling got left out of the bag as other lenses get used more.
2. Cheap lenses are good for learning. 50.18 is a great example. The image quality may not be the best wide open, but it's a great way to learn about DOF.
3. Lens prices go up, bodies go down. I've watched new (and used) lens prices rocket in the past couple of years. If I'd had the cash in 2007......
4. Try lenses out. The Buy/Sell forums here mean you can recover most of the cost if you decide you don't like it.
5. The L's are worth the money.
6. Eventually you will end up buying the lens(es) you initially lusted after!
Hope this is useful to people starting out...