I've found a good deal on a used 150/2.0 (less than a 40-150 Pro), and I'm a little tempted. The rational person in me says 'stick with the 50-200 SWD'. But there's real appeal to a very fast telephoto. How is the AF speed? Is it worth it? I would probably mostly use it on trips involving wildlife, and the weight (more than bulk) holds me back a little (and is the thing that rules out the 300/2.8). I'm also intruiged by the upcoming 100-400 (despite the slow long end) for its range, but a fast tele prime is something I've never owned yet. I'm wondering if it's really worth it. I have more or less decided that MfT will be my telephoto system and Sony for the wide end, and this is a very tempting offer... Argh!