I would look at buying the Canon 70-200mm f/4 L lens and a 50mm f/1.4 lens to fill the gap between 30mm and 70mm. The 70-200mm f/2.8 IS is a great lens, but the faster aperture and image stabilization aren't necessary if you're shooting landscapes.
I guess I'll have to repeat myself:
Yes, it can most certainly be necessary if you're shooting landscapes.
Landscapes aren't always shot at f/8 or f/11 with 1/1000 shutter time.
Sometimes, f/2.8 can give pleasing effects in landscapes.
I also quite often find myself shooting under adverse conditions where image stabilization really helps:
- strong winds (the wind will catch on your lens shade)
- hand-held landscape stunt photography (when there's no time to set up the tripod)
- with the 1.4x Extender
(I use the 20D, so the 1.6x FOV factor comes into play, too, just as it does for davidr805.)
I just wish that Canon had licensed the anti-shake system from KM, but I'm sure we'll see a dedicated mirror lockup button before then.
What is there to say about the Canon 70-200 f2 vs the Canon 70-200 f/2 IS
(They're both f/2.8, not f/2.)
In my opinion, the f/2.8 non-IS is pointless. There is just a small weight difference between the two lenses, and as far as I've been able to figure out without purchasing the other lens, the optical difference is negligible. The only thing the f/2.8 has got that may be important is a lower price, but if you can afford the IS version, I'd say go for it.
The canon 50mm would be a nice thing to have.
Yes, but how often will you be using it?
I've found that my 50mm often stays in the bag, but then again, I have the 24-70mm, so switching to the 50mm becomes a bit of a hassle for what is often just a small gain.
Get one of the 70-200 lenses, and make a note of how often you feel that you need something inbetween the 15-30 and the 70-200. If the problem doesn't occur, well, you've saved a few bucks. If not, the 50mm will still be available in a store near you.
Well, that's just my opinion, anyway.