Thanks dubcat! I haven't updated that in a while so check out my tumblr blog too, there are some XP1 images in there.
With the smaller cameras you don't get to see what the depth of field would be like (as you would with an SLR), so visually it feels more like looking through a window frame, so I feel more connected with the scene I'm photographing. Secondly, since the OVF and lens are separate, you don't have your viewfinder blacking out every time you release the shutter - you know exactly what moment you've captured when you press the shutter. You can also see what's happening outside the frame lines with the OVF too. A good way of imagining what framing with the OVF would be like is to use your index fingers and thumbs to make a rectangle and look through that. I wouldn't say it's better than viewing through the lens as they have their plus points too, I shot street on an SLR for years before moving to rangefinders/Fuji X's.
The X-Pro 1 has an option that marks out where the AF point will be below 1m in the OVF so you don't have to think about parallax too much. I think it'd be best for you to wait for the X100s though, the AF on that should be much better (so the XP2/XE2 should be much closer, if not on a par, with SLRs). I think the AF on the XP1/XE1 is acceptable, but I'm coming from a manual focus world - Fuji are supposed to be releasing a firmware update in the next week to make the AF even faster (I'll let you know what it's like). The camera has changed a lot in terms of speed since they released firmware 2.0 last year. I do shoot a lot of street on it though, if the AF really was horrible I would have returned it! I think the Sony NEX cameras are better in AF terms (their video mode is better too), you don't get the OVF of the XP1 but their EVFs are amazing (even if they don't have the advantages I mentioned above).
