I have been debating the purchase of the Canon 300 mm f2.8L IS for a while now. I finally took the plunge (I wish I had stock in B&H!).
So far, I could not be happier. I've had just a few opportunities to see what it can do, but the clarity is unrivaled, at least in my experience. In fact, sometimes I see almost too much detail. Check out the dandruff on this cormorant
For subjects without dandruff (and even for those that do!), the level of detail I can capture is opening up new possibilities. I had always admired the art deco tiles on this condo building, but didn't know how to capture them.
It's NOT all about the gear, but sometimes a lens is so useful it makes the job much, much easier. Now, I just need to find a good chiropractor for my shoudlers from carrying this thing around!