I've been using tungle.me for about a year now. The service lets users post their availability publicly (tungle.me/Alexey), letting users send emails like "Sure, let's get coffee next week. My availability's at tungle.me/Alexey; let me know what works for you." Availability data is taken from users' Google calendars; if you're like me and meticulously schedule your day on GCal, the remaining 'empty' slots coincide quite reasonably with times I'm available. Visitors to a tungle.me page also have the option to schedule a meeting directly on the page.
Review
All in all, the service is great. It solves so many 2-way scheduling problems that I ended up having my tungle.me page as the only thing in my email signature. The site still has a number of kinks - schedulers are forced to offer multiple meeting times, for example, and a failure to synchronize with GCal will result in me being listed as 'available' when I'm not). A couple of extra features would be nice, too; for example, tungle.me/Alexey/30-minutes-next-week should be a link I can send somebody that would limit potential meeting times to available 30-minute blocks next week. Still, tungle.me solves a time-consuming pain point, so I use it.
It's worth noting that what I've seen of the site's growth has been entirely viral. The company is based in Montreal; I found out about it when I got a Skype meeting invite through tungle.me. Since then, several friends have adopted the service. Tungle.me is viral.
Reaction
Mostly, people tend to react positively or neutrally to my sending out tungle.me links. Less tech-savvy users tend to continue to schedule meetings over email, (How about 6PM on Thursday? Your tungle.me says you're free) though I do get the occasional tungle-direct invite as well.
A more interesting (and rarer) response, however, has been to refuse to use the service and instead insist on coordinating schedules over email. Partly, this could be a direct Luddite response, a la "I don't trust new apps," (not that any of a user's data is being accessed or used). A more common issue is that of social status. Saying "You, go pick a meeting time for us" can be interpreted in two ways: either as "I really want to meet with you, pick any time you're free," or "Go talk to my digital secretary about scheduling a meeting; I don't deal with little things like that personally."
The second interpretation pops up in sitautions when you try to ask somebody for a meeting and then ask them to schedule it. The gut reaction is often "wait, you want this meeting - why am I doing the scheduling?" Rationally, using tungle.me is time-saving for both parties, but it's hard to explain that to somebody you're working your ass off trying to get a meeting with. Social dynamics are hard; norms may adjust to be more tungle.me friendly in the future, or tungle.me might find ways to make it look like the initiative is being taken by the meeting proposer.
Conclusion
tungle.me is well positioned amongst early adopters, viral, and useful. As it expands beyond early adopters, I think it's going to be part of the future of scheduling.
What do you think - am I being a douche by asking contacts to check out my availability on tungle.me instead of taking the time to write it in an email?
Note: Multi-person meeting schedulers (scheduleonce/when2meet/doodle) are interesting too and probably deserve a post of their own.