Arc is pretty good. The vertical tabs are great, and the same tab list appears in all other windows you have open. For someone who easily loses track of tabs, this helps dramatically, and prevents the situation where you have multiple windows open, each with similar tabs.

The other great feature is the "Little Arc" window that appears when opening links in other apps. This lets you check something out, close the window, and resume what you were doing, preventing you from getting sucked into the Web and away from the conversation you were having.

I don't find much else compelling, but these are both really nice. For some reason, I don't care at all about the tiling system or the Boosts feature (modifying pages to remove elements, change fonts, etc) even though people talk about those a lot. If they can think of one or two more really useful features (and communicate them properly on the website) then they'll gain a lot of users.

At least, they'll gain a lot of users on the Mac. The biggest downside is the lack of Windows and Linux support. They're working on Windows. I don't see them doing Linux at all, but who knows.