Voice over IP and SIP have evolved over the course of the last 10 years to the point that they have become a viable substitute for traditional business voice services. In fact, the vast majority of Fortune 1000 companies have been using this technology for nearly 10 years. Millions of smaller businesses have also transitioned to SIP in the last 2-3 years.
Since SIP Trunks utilize your existing broadband connection (whether it's cable, DSL or T1), the quality and reliability of the service are dependent upon the quality and the reliability of your broadband connection. Not all broadband connections are the same. Clearfly sets some minimum requirements in order for a customer to qualify for the service and provides you with an easy-to-use tool (available here) to assess whether or not your broadband connection meets these quality requirements.
Assuming your broadband's reliability and quality meet the requirements, you can determine how much bandwidth your SIP trunks will require here.