I've been using voip for about a year. I cancelled my plain ol' Verizon service a few weeks after I started using voip and I have never regretted it. I was paying around $37 a month for Verizon with caller ID included. Now I pay around $22 per month for voip and I get caller ID, voice mail, call forwarding (great for forwarding calls from telemarketers), 3-way calling, etc.
Not all of the VOIP providers offer 911 service, but that's supposed to change soon. I think they're gonna be required to offer it some time this year. If 911 is a concern, you can set up your phone's speed dial to call your local fire or rescue service. This won't tell them your location, but it's easier than digging up their number in an emergency.
What I would suggest you do is look into the companies that offer free trials, but read their terms carefully. Many offer free trials, but if you cancel you have to send back the equipment within a certain time period to get your money back. I tried Broadvoice and Vonage, both of which offered a trial period, before I decided to go with Broadvoice. Vonage did refund my money without problem. I'd recommend trying *at least* 2 companies using their free trials and determining which one is best. And check out the voip forum at dslreports.com for people's opinions on your potential voip provider. I think they also offer reviews of voip providers.
If there are people at your residence who are likely to require emergency medical service at any time, I would not yet recommend VOIP as your only service. With regular phone service, you only have to depend on one company (the phone monopoly). With voip, you have to rely on your cable company, your power company (unless you have backup in case of an outage), and your voip company to get your call across. I have not yet noticed Broadvoice outages, but stupid Adelphia sometimes drops my connection.
As for phones, you should be able to use *any* phone with your voip service, including corded or cordless. You may run into problems, though, if you use a wireless lan at home. If that's the case, you should use a phone that is on a different frequency (900mhz or 2.4 ghz or 5.8 ghz or whatever they are) to avoid interference.