What are the advantages and the disadvantages of using an ISP?

and their relative importance will depend on your needs. Head over to http://www.dslreports.com/ to get a feel for what is causing other customers pain. Here's a quick list of what factors to consider when evaluating ISPs:

1. Upfront and ongoing costs.
2. Upstream and downstream last-mile bandwidth (the speed from the ISP to your network).
3. Average wait time for support calls.
4. Average uptime (usually expressed as a percentage, such as 99.9%).
5. Static IP vs. Dynamic IP (with Static IP being better only if you host a server).
6. Security services, such as a firewall, filtering router, or intrusion detection.
7. The bandwidth the ISP has to other ISPs, and the redundancy of those connections.
8. Additional services that you may need, such as e-mail, Web hosting, and newsgroups.