What is interesting about Path's "With" product (see their blog) is that it leans on a very specific "trigger" compared to, say, Foursquare. The trigger is, "I'm out doing something social with my friends." This is much more specific and clear cut a scenario than "anytime I go somewhere" (Foursquare). It also communicates its usecase in a more compelling manner than a product like Color does.
It could be interesting to see how a product like this could tell us about how our friendships are changing (particularly in a negative way - "you are no longer seeing this person a lot") before we even begin to sense that that change is happening.
On the flipside, with a big enough adoption, something like this could have some interesting effects, and not necessarily in a good way. Social interactions definitely benefit from asymmetry (or at least expected asymmetry) of information. If you are a friend of mine and visit Palo Alto, I don't know about it, and we don't see each other, well, no harm, no foul. But, if I know about your visit, and then you don't call, it becomes problematic. Similarly, if, at the current moment, I get "busy" and we don't hang out for a few weeks, it's not generally a big deal if there's a sense that I just lost track of time. But, if I quantitatively "knew" through a service like With that I was "ignoring" you, would that lapse still be acceptable?
I can only imagine this situation would get worse if I also knew precisely who you were hanging out with instead of me due to the public nature of With checkins...
The "Quantified Self" movement has definitely been gaining steam, but it's interesting, using something like With as a thinking tool, to think about whether that movement would make sense in the social realm - or more likely, how to make sense in the social realm a certain approach would have to happen. And even if it does, how it might behave differently at scale. For now, I think With is a product I would use in its current stage - a quieter, smaller userbase that doesn't include all of my 600-odd Facebook friends. Still, I'm increasingly fascinated by the way this general trend towards the flattening of information asymmetry is changing the way we interact, starting with online but then reflecting in offline changes a well, and whether this trend is sustainable as it scales.
On an unrelated note, from a development standpoint, I love With's positioning as a "Path short." This means that it is an experiment that can begin to test and explore what users are doing, without changing their core service before it's necessary. Further, it creates an opportunity to build a new behavior within a user base that can then be incorporated into the larger product. Seems like a very good way to begin phrasing, exploring and innovating.
I'm not a Path user at this moment, but maybe this product is a good opportunity for Path to discover why, and learn more about me and my behavior.