We've generally found it to be decent, although (naturally) variable. We use Twilio alongside a UK SIP provider called Voipfone (http://voipfone.co.uk) which is how we actually get our calls through to PSTN.
Twilio Client (https://www.twilio.com/client) seems to provide better quality, but we really like the convenience of hardware phones and built-in PBX functionality.