I've heard people say all kinds of things about NTL but I've been with them for about 2 years now with no problems. They've got good uptime, they regularly upgrade my connection speed and it's relatively cheap. I have the £24.99 2 MB package. The 10MB package costs £34.99. I've just received an email saying that they'll be making mine unmetered sometime this month, which is great.
If you connect the modem via Ethernet instead of USB, it will work on a Mac (actually, if you install it via USB on Windows, you have to install their software which is pretty crappy).