I've been running asterisk on my linode for over a year now.

Everything works really well, except for conferencing, as someone else has mentioned.

Why choose linode for VoIP? Well, I already needed a VPS solution, and I choose linode. It just happens that I can also run asterisk on it at the same time, so, why not?

Would I use linode for a dedicated VoIP server? No. If I really needed to run failsafe VoIP for a client, I host internally with dedicated lines and servers.