i have been using no-ip.com for few years already and no problems so far. You friends being unable to connect to you using the dynamic domain is not because of using no-ip.com. To find out, try asking them to connect using direct ip the next time to see if they can connect.

They best dynamic domain is dyndns.org and no-ip.com. Don't waste time on others. If those two doesn't work then the problem is your side or your friend's side.

As for registering a full domain name for dynamic site. It's a real waste of money unless you are making some money out of that hosting.